Militant young squatters fired flare guns and hurled firebombs and rocks at police into early Tuesday after authorities cleared out three occupied houses in eastern Berlin.

The protesters, many wearing black hoods, shoved cars and huge metal trash bins into the middle of streets to act as barricades.Others smashed glass windows in shops and set cars on fire in the Friedrichshain neighborhood, formerly part of East Berlin.

An estimated 1,400 police responded with clubs, tear gas and truck-mounted water cannons in an attempt to disperse the 500 protesters, who continued the street battles for several hours.

Twenty of the militants were arrested and 160 police officers were injured, authorities said. The number of injured squatters was not known.

The militant squatters had taken over a strip of 12 abandoned buildings on Mainzer Street in recent months.

As more people pack into Berlin with the recent changes in Germany and Eastern Europe, there has been increasing need for affordable housing. Many apartments are now financially out of reach for ordinary workers and students.

Stunned eastern Berlin commuters watched the melee.

Tuesday, city spokesman Werner Thronicker described the clashes as "extremely serious."