Saakashvili lost his Georgian citizenship when he was granted a Ukrainian passport in 2015, as the country does not allow dual citizenship.
After crossing the border Saakashvili thanked his supporters, saying all of them were heroes.
Back home in Georgia, he started out as an internationally praised reformer but later he was criticised for monopolising power and exerting pressure on the judiciary.
Kiev is "panicking", Saakashvili said, adding that he did "not want to overthrow President Poroshenko" but just defend his rights.
The failure to stop Mr Saakashvili from returning was an embarrassment for Kiev and another headache for a leader fighting a Russian-backed insurgency and trying to restart a struggling economy. "This is very sad", Saakashvili said Friday in Warsaw.
Their relationship dates back almost three decades to when they were students at the same university in Kiev and their shared opposition to the Russian government brought them together as politicians.
Saakashvili said the decision was politically motivated. Saakashvili gave up his Georgian citizenship to take the job, where he rapidly began sweeping anti-corruption activities.
"But instead, he committed a crime - because the state border must not be violated", Poroshenko said. I, as president, will surely comply with any court ruling.
"They swept us up and carried us into Ukraine", Saakashvili said.
Ukraine's record of implementing reforms has been patchy since Poroshenko took office in 2014. "I don't think this issue (Saakashvili's travel) is qualified as a serious one".
"We didn't want this country when we stayed on Maidan", he told reporters.
"It looks like [Poroshenko] is getting rid of a political opponent".
Saakashvili and a crowd of supporters are proceeding into Ukraine on foot after breaking through a line of guards on the Polish-Ukrainian border.
Kiev could leave Saakashvili alone, arrest him and possibly extradite him to Georgia.
Georgia has pursued Saakashvili on criminal charges of abuse of power and misappropriation of property. The 49-year-old is now wanted on criminal charges in Georgia, which he says were trumped up for political reasons.