Hooded youths and pro-Palestinian demonstrators clashed with police as President Bush arrived here Wednesday, marring a peaceful protest by 20,000 people opposed to any widening of the U.S. war on terrorism.

Pro-Palestinian demonstrators waving Palestinian flags and wearing the checkered Arab keffiyeh headdress burned American flags at a former Prussian parade ground, where earlier demonstrators had sunbathed in a picnic-like atmosphere.

Nearby, youths pelted police in riot gear with bottles and stones.

Plainclothes security officials isolated troublemakers, removing individuals from the crowd as thousands of peaceful protesters scattered or watched in dismay from the steps of the Berlin cathedral and Altes Museum.

Riot police later used water canons to disperse the remaining crowd and violent demonstrators smashed windows at a McDonald's restaurant and department store on Alexanderplatz.

Police confirmed arrests but had no immediate word how many people had been taken into custody. Several ambulance took away injured people, but no numbers of injured were available.

Bush was at least half a mile from the violence, meeting with Chancellor Gerhard Schroeder and Berlin Mayor Klaus Wowereit at a restaurant adjacent to the Brandenburg Gate at the opposite end of the famed Unter den Linden boulevard.

A contingent of 10,000 police were deployed to control access to Berlin's government center, where Bush will spend the night, meet with German leaders and address parliament during his brief stay.

The anti-war demonstration was organized by an umbrella group of pacifists and anti-globalization groups who vowed to keep the demonstration peaceful so as not to deflect from their message.

But they were undermined by the pro-Palestinian and anti-capitalist elements that converged on police as the larger group ended its daylong peaceful gathering to protest what they saw as a U.S. tendency to go it alone on the world stage.

Waving banners calling the war on terrorism an "imperialist crusade," demonstrators at the main protest cited fears that Washington will expand its military campaign to Iraq as well as the U.S. abstentions from the international criminal court and the Kyoto protocol on climate change.

"It's not against America or the American people, but against the Bush government, which I think is very radically unilateralist," said 59-year-old Hrvoje Jurcic, a librarian from Bavaria.

Pretzels bobbed from sticks as the crowd marched around Alexanderplatz, taunting the president who in February fell unconscious after reportedly choking on a pretzel while watching the Superbowl. Banners read: "Pretzels, not bombs."

The umbrella group of organizations calling itself the "Axis of Peace" organized similar protests across Germany.

Bush was relaxed about the protests, telling European reporters in Washington before his departure, "That's democracy."

Schroeder also emphasized the right to protest — but urged demonstrators to recall America's contribution to securing West Berlin during the Cold War and to allowing German reunification.

"I think one should consider that this right was guaranteed in the free part of Germany by American soldiers during the Cold War," Schroeder told Germany's N-24 television.

Bush will face a skeptical audience as he outlines what German government officials described as a far-reaching counterterrorism plan in a speech to the Bundestag on Thursday — not only among the German public but some German lawmakers, who aired their misgivings over a possible U.S. military strike against Iraq.

"Military action against Iraq is not justified as long as isn't certain that that Saddam supports or shelters Al Qaeda terrorists," said Peter Struck, the leader in parliament of Chancellor Gerhard Schroeder's Social Democrats.