Mr. Saakashvili responded in a post on his Facebook page, saying that Mr. Poroshenko had “crossed a red line” with the decree, and by failing to address corruption.

He said the president was risking stirring another popular uprising in Ukraine. And in the ultimate insult for opponents of the Russian president, Vladimir V. Putin, Mr. Saakashvili suggested that Mr. Poroshenko might have to flee to Russia.

“Now, doubtless, you, as was the case with your predecessor, will be tempted to try and hold on to power at any price,” he wrote. “You may try this, just remember, they are waiting for you in Russia! Ukrainians have twice dealt with this pestilence and they will not stop.”

Mr. Poroshenko’s office did not publish the decree stripping his onetime ally of citizenship, and there was some confusion over the legal basis for the action. Local media quoted officials saying that Mr. Saakashvili had failed to disclose on his citizenship application that he was the subject of a criminal investigation in Georgia.

The decree sent ripples through Ukrainian politics. The Georgians who moved to Ukraine to join the government after 2014 have been a powerful force behind the scenes, working to overhaul the country’s bureaucracy. To comply with Ukrainian law on government service, they had to renounce their Georgian citizenship and accept Ukrainian citizenship.