Police identified the man as 44-year-old Maksym Kryvosh, born in Russia
A man who said he was armed with weapons and explosives seized a bus in the western Ukrainian city of Lutsk on Tuesday and took a number of people on board hostage, police said.
Ukrainian President Volodymyr Zelenskiy said "shots were heard, the bus was damaged". Police blocked off the city centre.
Police identified the man as 44-year-old Maksym Kryvosh, born in Russia.
They said in a statement that he had said he was armed with explosives and weapons. Between 10 and 20 people were onboard the bus, they said.
In posts on social media, he demanded that senior Ukrainian officials publish statements saying that they were terrorists. He also threatened to detonate another bomb in a crowded place.
Deputy interior minister Anton Gerashchenko said Kryvosh had been convicted twice and spent about 10 years in prison.
Streets in Lutsk were cordoned off by uniformed personnel, police cars and an armoured personnel carrier.
"We are in full control of the situation. I know all the details. I am talking with our specialists who are in Lutsk. Professionals are working, doing everything to free our hostages," Zelenskiy's press service quoted him as saying.
Photos showed a small bus parked in the middle of an empty street. Two windows of the bus were smashed and other windows were covered with curtains.