They tore down sheets of plywood nailed up by shopkeepers to protect store windows, and then fed the wood to the burning barricades.

Policemen with helmets, plastic shields and clubs responded to the attacks by firing water cannons and tear gas canisters at the demonstrators. Charging policemen drove the demonstrators back.

Convoys of police trucks roared through the square. The air was thick with tear gas and black smoke from burning automobiles. Police loudspeakers, mounted on trucks, announced the location of checkpoints where demonstrators could leave the square. Some demonstrators managed to reach side streets, where they threw rocks at the police. Officers responded with water cannons and clubs. Clashes also occured in other parts of the city. On West Berlin's fashionable Kurfurstendamm, there were pitched battles between policemen and youths, and some store windows were smashed.

Fires were set and the Fire Department declared a state of emergency this afternoon. Police officials said there were 250 arrests by late afternoon. Five policemen were injured. There was no information on the number of demonstrators hurt.

In another demonstration, several thousand feminists in black, some with their faces painted, played drums, cymbals and castanets as they marched through the inner city to protest Mr. Reagan's presence.

Although the demonstration at the Nollendorfplatz was sponsored by the Alternative List and other political groups, police officials said it was dominated by radicals on the fringe of the Baader-Meinhof movement.

Radical magazines and newspapers available in leftist bookshops throughout the city printed manuals for behavior at the demonstration.