Former Ukrainian President Petro Poroshenko, a rival of current leader Volodymyr Zelensky, announced on Saturday that he had been refused leave of the country for a trip to Lithuania, criticizing a decision that broke the "political ceasefire" by full Russian invasion.

Mr. Poroshenko, president from 2014 to 2019 of Ukraine, is today at the head of the European Solidarity party, the second largest party in Parliament and in opposition to Mr. Zelensky.

According to his press service, Mr Poroshenko "was refused to cross the border into Ukraine" when he was to take part in the NATO parliamentary assembly in Vilnius and had received "all formal permissions to leave the country" as a permanent member of the Ukrainian delegation.

The former president was also due to meet in Vilnius with Lithuanian President Gitanas Nauseda and a group of Lithuanian and European parliamentarians. He was then to travel to Rotterdam in the Netherlands for a summit bringing together European parties.

“There is a risk that by this decision the authorities have broken the political ceasefire in place during wartime with the opposition forces, which is one of the pillars of national unity in the face of Russian aggression," said its press service.

The upheavals in Ukrainian politics had ceased since Russia invaded the country on February 24, with most political forces united in the defense of the country. Eleven parties deemed pro-Russian have been banned.