An armed man has taken about 20 people hostage on a bus in the western Ukrainian city of Lutsk, officials say.
Shots have been fired, police said, but there have been no reports of casualties so far. The man's demands are not clear.
Deputy Interior Minister Anton Herashchenko said the man claimed to have a bomb, reports BBC.
"The police launched a special hostage operation. Negotiations are under way," Mr Herashchenko said on Facebook.
Counter-terrorism officers have surrounded the bus and cordoned off the city centre, reports said. Nearby residents have been told not to leave their homes or offices.
Police spokesman Yurii Kroshko said it was still unclear what the man wanted.
"We are trying to negotiate, we are phoning him. He answered the phone earlier [but] he did not answer the next few attempts."
Ukrainian media have shared what appears to be the man's Twitter account, on which he is posting criticism of political figures.
However, the police spokesman said the posts contained no clear demands.
"These are just general phrases, general dissatisfaction with some system," he said.
Ukrainian President Volodymyr Zelensky said he was closely following the situation.
"Every effort is being made to resolve the situation without casualties," he said.