Police say all of the people taken hostage at a post office in Ukraine's second-biggest city have been released and the man who held them arrested.

Ukrainian police chief Serhiy Knyazev says no one was injured in the standoff in Kharkiv.

Police say the man said he had firearms and explosives, but made no immediate demands.

Police didn't say how many hostages there were. They said earlier that three women and two children were released following talks. A boy interviewed in a video posted by a Ukrainian journalist said there were seven hostages altogether. He said the man said he had a brain tumor and treated the hostages well, offering them tea or coffee.

Ukrainian television stations broadcast footage of the man being escorted out of the building by police.

In a Facebook statement, President Petro Poroshenko has thanked police and the national security service for taking part in the operation.