By Rob Strybel

WARSAW (Reuters) - Polish police say they mistakenly fired live ammunition, not rubber bullets, to quell a campus riot, killing one and seriously injuring several others.

Several top police officials have resigned and the central Polish city of Lodz announced three days of mourning following a night of clashes involving an armed gang, students and police which left some 70 people injured and led to 19 arrests.

Police say the disturbance erupted when up a gang of about 100 men armed with chains, bricks and metal rods, crashed university celebrations marking the end of the academic year.

"Never in the history of Lodz has there been a hooligan incident with so many victims," city mayor Jerzy Kropiwnicki told a televised news conference on Sunday.

"To set an example, those guilty must be indicted, tried and punished," Kropiwnicki said, adding that police incompetence only made the situation worse.

Prosecutors are investigating how real bullets were mistakenly used by police to restore order, killing a 19-year old man, critically injuring a 23-year old woman and wounding at least two others.

The city's two top police officials have quit and the regional commander also tendered his resignation on Sunday.

"Somehow 25 lead-tipped bullets got mixed in with the rubber bullets, six of which were fired and three struck the people who suffered in the incident," Lodz police spokesman Witold Kozicki told Reuters by telephone.

Police said they did not fire the remaining live bullets, after realising their mistake.

The Lodz shooting occurred a week after a still unresolved incident in the western city of Poznan in which a young motorist, mistaken for a wanted criminal, was shot dead by undercover policeman during a raid.