Mikheil Saakashvili, Georgia's ex-president and one-time regional governor in Ukraine, has crossed into the country, helped by hundreds of his supporters.
Mr Saakashvili said he was unexpectedly swept over the border by a crowd, angry that the crossing was closed.
"They swept us up and carried us into Ukraine," he said.
Ukrainian officials say he entered illegally, and 16 border guards and National guardsmen were injured.
There were scuffles at the border between Mr Saakashvili's supporters and border officials, Ukraine says.
Mr Saakashvili, formerly Georgian, then Ukrainian, is now a stateless person, as his Ukrainian citizenship was removed by his former ally, President Petro Poroshenko, after a falling out.
He is also wanted in Georgia on criminal charges, which he claims are politically motivated.
Earlier on Sunday, his train was held at a railway station in Przemysl, Poland, as Ukrainian border guards denied him entry.
Mr Saakashvili was joined by a number of his supporters, including former Ukrainian Prime Minister and current opposition leader Yulia Tymoshenko.
He later told a pro-opposition TV station that the border authorities "provoked and provoked" people.
But he praised the Ukrainian border guards and National Guard members for not using violence.
Mr Saakashvili, who was born in Georgia, has said he wants to return to his adopted Ukraine to contest President Poroshenko's decision to strip him of his citizenship, in July while he was out of the country.
In 2015, he was appointed governor of Odessa by Mr Poroshenko, but the two fell out last November after Mr Saakashvili accused the president of blocking efforts to stop corruption.
But in accepting Ukrainian citizenship, he surrendered his Georgian citizenship.
"The reality is for me today that the Georgian passport means guaranteed imprisonment for me in Georgia," he told the BBC at the time.
Once in Ukraine, he could possibly be arrested and deported to Georgia to face charges, the BBC's Europe Regional Editor, Danny Aeberhard, said.
But Mr Saakashvili has been adamant about his return to rally political supporters, having announced his return to the country as far back as July.
Ukraine is a signatory to the UN Convention on the Reduction of Statelessness, which prohibits the withdrawal of citizenship when doing so would result in a person becoming stateless.
Exceptions exist in limited circumstances such as fraud or disloyalty to the state, though it is not clear if either apply in Mr Saakashvili's case.
Ukraine's migration service said the president takes decisions on who is stripped of Ukrainian citizenship based on the conclusions of the citizenship commission.
It did not provide the exact reason, but stated that this could be done if a Ukrainian national acquired citizenship of another country or submitted false documents.