An armed man has seized a long-distance bus and taken several people in it hostage in northwestern Ukraine, launching an hours-long stand-off with police.
About 10 people are being held hostage, Ukraine's Security Service said in a Facebook statement.
Police earlier had put the number at 20. It was unclear how many people had been on board to start with, and whether any escaped.
Police sealed off the centre of Lutsk, a city 400km west of the capital Kyiv.
The assailant is armed and carrying explosives, police said. They said he threw a hand grenade that failed to explode at officers and shot at them at least twice during the still-ongoing siege.
Ukrainian President Volodymyr Zelenskiy said negotiations with the man are under way.
"We're doing everything to free the hostages," Zelenskiy told reporters.
The man called the police himself at 9.25 am on Tuesday after taking control of the bus and introduced himself as Maksim Plokhoy, Deputy Interior Minister Anton Gerashchenko said on Facebook.
In a Telegram account reportedly belonging to him, Plokhoy apparently admitted to taking people on the bus hostage, said that "the state has always been and always is the first terrorist" and demanded that top Ukrainian officials release statements on their social media pages calling themselves terrorists.
According to Gerashchenko, police have identified the man as Maksim Krivosh, a 44-year-old Ukrainian born in Russia.
Krivosh had allegedly been convicted twice on a wide range of charges - robbery, fraud, illegal arms handling - and spent a total of 10 years behind bars.
Interior Minister Arsen Avakov travelled to Lutsk to supervise operations.
Ukrainian media reported that gunshots were regularly heard at the scene. It wasn't immediately clear whether anyone has been injured.
According to reports, Krivosh tried to reach out to journalists through hostages and their phones, demanding that they spread the word about his demands and get more reporters to arrive to the scene.
Zelenskiy said earlier on Tuesday he is personally monitoring the situation.