Ukrainian police are involved in a tense confrontation with an armed man who took around 20 people hostage on a bus in the western town of Lutsk.
Officers said they came under fire and a grenade was thrown, although it did not explode.
The man was named by police Maksym Kryvosh, 44, from Russia who has already been convicted and underwent psychiatric treatment.
It is said to demand that senior politicians declare themselves terrorists.
Deputy Interior Minister Anton Herashchenko said police were speaking with the man in the hope of resolving the crisis “through negotiations”.
Photos from the scene showed agents from the SBU’s counterterrorism security services surrounding the bus. Police said two shots were fired from it and the man also claimed to have explosives.
“The attacker threw a grenade from the bus, which fortunately did not explode,” a statement read.
There is no word on the condition of the hostages, who have been on the bus for several hours.
According to the attorney general’s office, the shooter said he placed an explosive device in a public place in the city that could detonate from a distance.
Police cordoned off the city center and told residents not to leave their homes or offices.
Mr Herashchenko said Kryvosh had spent around 10 years in prison for convictions, including fraud and illegal handling of weapons.
Ukrainian media shared what appears to be the man’s Twitter account, on which he posts reviews of national figures.
President Volodymyr Zelensky said he was following the situation closely.
“We are in total control of the situation. I am speaking with our specialists who are in Lutsk. The professionals are working, doing everything to free our hostages, ”he said.