At least 20 people have died in clashes between Ukrainian forces and pro-Russian rebels in the southeastern port city of Mariupol, according to Ukrainian interior minister Arsen Avakov.

"About 20 terrorists killed, 4 captured. A considerable part of the assailants have abandoned weapons and disappeared into uptown," Avakov wrote on Facebook. "Our losses: one killed and 5 wounded."

According to Interfax-Ukraine news agency, police were trying to take control of the police building when they were assaulted by pro-Russia separatists who had taken the interior ministry headquarters.

Mariupol police station caught fire and was pictured burning fiercely by ITV News foreign news editor John Angier:

A producer for Ruptly, an international video news agency and part of RT network, was shot in the stomach and taken to the hospital.

Ukrainian troops used large-calibre weapons in an attempt to take back an occupied government building. According to witnesses, heavy guns were used.

The fighting took place as Ukraine commemorated Victory Day celebrations marking the Soviet defeat of German Nazi forces in World War II.