Kiev, July 21 : Police are engaged in a tense stand-off with an armed man who has taken about 20 people hostage on a bus in the western Ukrainian city of Lutsk on Tuesday. Officers said they had come under fire and a grenade was thrown, although it did not explode. The man has been named by police as Maksym Kryvosh, 44, from Russia who has previous convictions and has undergone psychiatric treatment, the BBC reported. He is said to be demanding that senior politicians state they are terrorists. Deputy Interior Minister Anton Herashchenko said police were talking to the man in the hope of resolving the crisis "through negotiations". Pictures from the scene showed officers of the counter-terrorism SBU security services surrounding the bus. Police said two shots had been fired from it and the man claimed to also have explosives. "The attacker threw a grenade from the bus, which, fortunately, did not detonate," a statement said. There is no word on the condition of the hostages, who have been on the bus for several hours. According to the prosecutor general's office, the gunman says he has placed an explosive device in a public place in the city that could be detonated remotely. Police have cordoned off the city centre and told residents not to leave their homes or offices. Herashchenko said Kryvosh had spent around 10 years in prison on convictions including fraud and illegal handling of weapons. Ukrainian media have shared what appears to be the man's Twitter account, on which he is posting criticism of senior national figures. President Volodymyr Zelensky said he was closely following the situation. "We are in full control of the situation. I am talking with our specialists who are in Lutsk. Professionals are working, doing everything to free our hostages," he said. /IANS