KYIV, Ukraine — Ukraine’s former president and a leading opposition figure, Petro O. Poroshenko, returned Monday to Kyiv, where he faced possible arrest, adding internal political turmoil to the mounting threat of a Russian invasion.

Mr. Poroshenko’s return brought into focus Ukraine’s wobbly politics, which were mostly in the background in recent weeks as the United States and its allies in Europe scrambled to forestall Russian military intervention.

He arrived Monday morning at Kyiv’s Zhuliani airport, where a scene erupted at passport control. Mr. Poroshenko said border guards for some time refused to allow him to enter the country, though he was due to appear at a court hearing that day in Kyiv. He later passed the border control but said authorities had confiscated his passport.

Ukraine’s president, Volodymyr Zelensky, has been embroiled in a long-running feud with Mr. Poroshenko, who was president from 2014 to 2019. Since Mr. Zelensky took power, Mr. Poroshenko has been questioned as a witness in a raft of criminal cases that critics say were politically motivated.

Mr. Poroshenko appeared in court later Monday on charges of high treason and supporting terrorism, but by late in the evening in Kyiv, the court had not issued a ruling.

His appearance in the capital where he once governed comes after a week of mostly futile negotiations between Russia and the West seeking a solution to tense disagreements over the security of Eastern Europe.

In an interview before his return to Ukraine, Mr. Poroshenko said that his arrest might help Mr. Zelensky sideline a rival but that the political instability would play into the hands of President Vladimir V. Putin of Russia.

“He wants to undermine the stability in Ukraine,” Mr. Poroshenko said of Mr. Putin. “He analyzes two versions: One version is a military aggression through the Ukrainian-Russian or Ukrainian-Belarusian border. The second is just to undermine the stability inside Ukraine, and in this way just stop Ukraine from our future membership in NATO and in the E.U.”

In Kyiv, opinions differed on whether the threat of an arrest was just another maneuver in Ukraine’s typically byzantine politics at home, or something more ominous related to the Russian threat.

Analysts suggested that Mr. Zelensky might be seizing on the distraction of the Russian military buildup on the Ukrainian border to sideline an opponent, or that he hoped to tamp down possible opposition protests if he is forced to make unpopular concessions to Moscow to avoid an invasion.

“Maybe he thinks that with forces on the border, Ukrainians won’t protest” an arrest of the opposition leader, said Volodymyr Yermolenko, editor in chief of Ukraine World, a journal covering politics. If so, he said, it is a risky move.

“With the situation on the border, when everybody is yelling, ‘There will be a war,’ it’s very strange,” Mr. Yermolenko said of the spectacle of Ukraine’s two leading politicians squabbling despite the existential threat to their country. “It just seems ridiculous.”

Polls have consistently shown Mr. Zelensky and Mr. Poroshenko to be Ukraine’s most popular politicians. Mr. Poroshenko has a base of support in Ukrainian nationalist politics, particularly in the country’s western regions, which want closer ties with Europe, and he has criticized Mr. Zelensky for giving ground in peace negotiations with Russia to resolve the war in eastern Ukraine.

Mr. Poroshenko left Ukraine last month, saying he had meetings in Europe. Prosecutors say he left to avoid a court hearing.

Mr. Zelensky’s aides have said that the charges against Mr. Poroshenko are justified and that courts decided the timing of the arrest and other actions, including the freezing of Mr. Poroshenko’s assets earlier this month.

The former president was accused of missing a court hearing last month while traveling abroad. He returned to Ukraine on Monday despite reports in the Ukrainian news media that a court had issued a sealed order for his arrest.

Mr. Poroshenko left the presidency in 2019, when he lost an election to Mr. Zelensky, a former comedian who ran as an outsider to politics who would fight corruption and uproot the entrenched interests of Ukraine’s political class. Mr. Zelensky’s popularity has since slumped. Opinion polls today show only a slight advantage in a potential future election against Mr. Poroshenko, who is now a member of Parliament in the European Solidarity party.

Mr. Poroshenko offered no evidence of a Russian hand in the political turmoil and described internal Ukrainian feuds as the most likely cause of the legal pressure he faced. But he said Mr. Zelensky might hope to win concessions from Russia by arresting a politician aligned with the nationalist wing of Ukrainian politics.

“I am absolutely confident this is a very important gift to Putin,” Mr. Poroshenko said. “Maybe with this gift he wanted to launch a negotiation with Putin, as a precondition.”

After massing tens of thousands of soldiers on Ukraine’s border through the fall, Russia demanded last month that the United States and NATO pull back forces from countries in Eastern Europe and guarantee that Ukraine not join the Western alliance.

Diplomatic talks last week with Russia ended inconclusively, and Russian officials now say they are awaiting a written response to their demands from the United States.

As a contingency, in case diplomacy fails, Ukraine has also been quietly pursuing talks with Russia and proposed a bilateral meeting between Mr. Zelensky and Mr. Putin. On Friday, the Ukrainian presidential chief of staff, Andri Yermak, suggested a three-way video conference with the Russian and Ukrainian leaders and President Biden.

Mr. Poroshenko’s controversial return was not the first sign of political turmoil. In November, just as Russia was ramping up its deployments along the border, Mr. Zelensky told journalists that Russia was also planning a coup.

He said Russian operatives were seeking to draw one of Ukraine’s wealthy businessmen, Rinat Akhmetov, into a plot against his government. The businessman was “being dragged into a war against the Ukrainian state,” Mr. Zelensky said, but he provided no evidence and made no move to arrest Mr. Akhmetov.

Mr. Akhmetov vehemently denied any involvement in a plot to undermine Mr. Zelensky’s government.

KYIV, Ukraine — Ukraine’s former president and a leading opposition figure, Petro O. Poroshenko, returned Monday to Kyiv, where he faced possible arrest, adding internal political turmoil to the mounting threat of a Russian invasion.

Mr. Poroshenko’s return brought into focus Ukraine’s wobbly politics, which were mostly in the background in recent weeks as the United States and its allies in Europe scrambled to forestall Russian military intervention.

He arrived Monday morning at Kyiv’s Zhuliani airport, where a scene erupted at passport control. Mr. Poroshenko said border guards for some time refused to allow him to enter the country, though he was due to appear at a court hearing later in the day in Kyiv. He later passed the border control but said authorities had confiscated his passport.

Ukraine’s president, Volodymyr Zelensky, has been embroiled in a long-running feud with Mr. Poroshenko, who was president from 2014 to 2019. Mr. Poroshenko faces a court hearing late Monday morning on charges of high treason and supporting terrorism.

His appearance in the capital where he once governed comes after a week of mostly futile negotiations between Russia and the West seeking a solution to tense disagreements over the security of Eastern Europe.

In Kyiv, opinions differed on whether the threat of an arrest was just another maneuver in Ukraine’s typically byzantine politics at home, or something more ominous related to the Russian threat.

Analysts suggested that Mr. Zelensky might be seizing on the distraction of the Russian military buildup on the Ukrainian border to sideline an opponent, or that he hoped to tamp down possible opposition protests if he is forced to make unpopular concessions to Moscow to avoid an invasion.

“Maybe he thinks that with forces on the border, Ukrainians won’t protest” an arrest of the opposition leader, said Volodymyr Yermolenko, editor in chief of Ukraine World, a journal covering politics. If so, he said, it is a risky move.

“With the situation on the border, when everybody is yelling, ‘There will be a war,’ it’s very strange,” Mr. Yermolenko said of the spectacle of Ukraine’s two leading politicians squabbling despite the existential threat to their country. “It just seems ridiculous.”

Polls have consistently shown Mr. Zelensky and Mr. Poroshenko to be Ukraine’s most popular politicians. Mr. Poroshenko has a base of support in Ukrainian nationalist politics, particularly in the country’s western regions, which want closer ties with Europe, and he has criticized Mr. Zelensky for giving ground in peace negotiations with Russia to resolve the war in eastern Ukraine.

Mr. Poroshenko left Ukraine last month, saying he had meetings in Europe. Prosecutors say he left to avoid a court hearing.

Mr. Zelensky’s aides have said that the charges against Mr. Poroshenko are justified and that courts decided the timing of the arrest and other actions, including the freezing of Mr. Poroshenko’s assets earlier this month.

The former president was accused of missing a court hearing last month while traveling abroad. He returned to Ukraine on Monday despite reports in the Ukrainian news media that a court had issued a sealed order for his arrest.

Mr. Poroshenko left the presidency in 2019, when he lost an election to Mr. Zelensky, a former comedian who ran as an outsider to politics who would fight corruption and uproot the entrenched interests of Ukraine’s political class. Mr. Zelensky’s popularity has since slumped. Opinion polls today show only a slight advantage in a potential future election against Mr. Poroshenko, who is now a member of Parliament in the European Solidarity party.

In an interview before his return to Ukraine, Mr. Poroshenko said that his arrest might help Mr. Zelensky sideline a rival but that the political instability would play into the hands of President Vladimir V. Putin of Russia.

“He wants to undermine the stability in Ukraine,” Mr. Poroshenko said of Mr. Putin. “He analyzes two versions: One version is a military aggression through the Ukrainian-Russian or Ukrainian-Belarusian border. The second is just to undermine the stability inside Ukraine, and in this way just stop Ukraine from our future membership in NATO and in the E.U.”

Mr. Poroshenko offered no evidence of a Russian hand in the political turmoil and described internal Ukrainian feuds as the most likely cause of the legal pressure he faced. But he said Mr. Zelensky might hope to win concessions from Russia by arresting a politician aligned with the nationalist wing of Ukrainian politics.

“I am absolutely confident this is a very important gift to Putin,” Mr. Poroshenko said. “Maybe with this gift he wanted to launch a negotiation with Putin, as a precondition.”

After massing tens of thousands of soldiers on Ukraine’s border through the fall, Russia demanded last month that the United States and NATO pull back forces from countries in Eastern Europe and guarantee that Ukraine not join the Western alliance.

Diplomatic talks last week with Russia ended inconclusively, and Russian officials now say they are awaiting a written response to their demands from the United States.

As a contingency, in case diplomacy fails, Ukraine has also been quietly pursuing talks with Russia and proposed a bilateral meeting between Mr. Zelensky and Mr. Putin. On Friday, the Ukrainian presidential chief of staff, Andri Yermak, suggested a three-way video conference with the Russian and Ukrainian leaders and President Biden.

Mr. Poroshenko’s controversial return was not the first sign of political turmoil. In November, just as Russia was ramping up its deployments along the border, Mr. Zelensky told journalists that Russia was also planning a coup.

He said Russian operatives were seeking to draw one of Ukraine’s wealthy businessmen, Rinat Akhmetov, into a plot against his government. The businessman was “being dragged into a war against the Ukrainian state,” Mr. Zelensky said, but he provided no evidence and made no move to arrest Mr. Akhmetov.

Mr. Akhmetov vehemently denied any involvement in a plot to undermine Mr. Zelensky’s government.