Each side had its script and its props. Each performed its role to perfection. It wasn't a play acted out, however, but a riot.

More than 40 people - including a dozen police officers - were injured and 246 rioters were arrested, police said.Berlin police and a veteran radical group re-enacted yet another of their violent confrontations, only this time it took place in what was once East Berlin.

Thousands of leftist radicals opposed to Wednesday's German unification used the first full day of the historic merger to rampage through East Berlin's sprawling central square, Alexanderplatz.

The radicals trashed outdoor concession stands, destroyed cars, smashed windows, hurled rocks and fired flare guns at police.

Police in riot gear used water cannons and tear gas against the radicals. The two sides battled for hours for control of the square, taking turns charging each other well into the night.

About 500 leftists and 200 Turkish youths took part in the riots, police said.

"It's almost comical," said East Berliner Herbert Schneider. He watched, dumbfounded, as running gangs of radicals and hooligans were chased around the square by hundreds of police with helmets, shields and nightsticks.

"We have a complicated future here," he said.

The clash began as a huge protest against German unity. The leftists gathered in a grassy square in what was West Berlin for a rally against what they said was rising racism and nationalism in both sides of Germany.

About 20,000 demonstrators marched into East Berlin, to a square just outside of Alexanderplatz, where they tore down the new West German flags from six flagpoles and hoisted the red-and-black flag of their anarchist movement and old Communist East German flags.

The protest turned violent when the leftists began smashing windows and trashing cars. Police ordered thousands of other people celebrating unity to leave the area, and the leftists eventually took over Alexanderplatz.

Each side took turns charging and retreating, repeating in near exact form past clashes that have been played out for many years in West Berlin.

The rioters joked and raided beer stands. "To unity," laughed a young woman after police briefly drove the thousands of people, most of them young West Berliners, to the square's perimeter.

A man who tried to drive through a swirling crowd of youths angrily stopped his car and challenged them. Police gently escorted him back to his vehicle.

East Germany has been marred by right-wing and left-wing extremist violence since it opened its borders to the West. But Wednesday was the first time West Berlin leftists staged one of their patented, hours-long, give-and-take riots.

Leftists had told The Associated Press earlier this week that the march against unity would turn into a clash with police. Police likewise had said that unity day celebrations would be marred by extremists.

The predictions came true.