By PATRICK REEVELL, ABC News

(MOSCOW) — Ukrainian police are in a tense standoff with a heavily armed man who has taken approximately 20 people hostage aboard a passenger bus in the town of Lutsk in western Ukraine.

Special forces police have cordoned off the bus in a central square of the city and authorities said they are holding negotiations with the gunman, who has claimed he has a large quantity of explosives with him.

Ukraine’s Interior ministry said the hostage-taker has warned them he has rigged the bus to explode and that another bomb was planted in Lutsk that he claims he could detonate remotely. Shots were fired early on in the standoff and police said a grenade was thrown from bus, though it didn’t explode. So far there have been no casualties reported.

The stand-off began mid-morning after the gunman called police saying he was hijacking the bus and taking the passengers hostage, Ukraine President Volodymyr Zelenskiy said in a statement. It is unclear when the shots were fired or by whom. Video from the scene showed the bus with two broken windows and heavily armed police lying on the ground a few meters away with rifles pointed at the bus. The exact number of people aboard the bus is not known but police have said it is between 10 and 20.

Authorities have named the hostage-taker as Maksym S. Kriyvosh, 44, originally born in Russia. A deputy Interior Minister Anton Gerashchenko said in a Facebook post that the gunman had introduced himself to police under the pseudonym “Maksim Plokhoy.”

Several social media accounts under that name have posted texts that include demands that top Ukrainian government and judicial officials, religious figures and several well-known oligarchs publish videos on YouTube where they would call themselves “terrorists.”

In the posts, there is a warning that if the video statements are not made then bombs will go off. Several of the posts are titled “Happy Anti-System Day.”

A Twitter account with the gunman’s name continued tweeting after the bus was seized, listing the demands. Twitter deleted the account a short time after.

Local media have reported that Kryvosh’s parents had lived in Lutsk. According to Ukraine’s Interior Ministry, Kriyvosh has previously been convicted twice by a court in the city in 1994 and 2005 of serious crimes, including fraud and illegal possession of arms and explosives. He has spent a total of 10 years in prison, Gerashchenko said.

President Zelenskiy wrote on his Telegram channel that authorities are doing everything they can to resolve the situation without casualties.

Ukraine Interior Minister Arsen Avakov, who traveled to Lutsk to oversee the police operation, told reporters that the situation remained difficult and urged journalists to be careful what they published.

A few hours after the stand-off began, authorities in Ukraine’s capital Kyiv said police had disarmed two explosive devices at a market in the city. Local media reported that several more bomb threats were also called into public buildings in Kyiv and the eastern city Kharkiv. Avakov, however, told reporters that the explosives found in Kyiv were not connected to the hostage-taking in Lutsk.

“Everyday we find explosives, some kind of weapons,” he said. “It’s doesn’t have any relation to this.”

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