Black-clad protesters attacked police and set a hotel and a customs station ablaze Saturday near a bridge linking France and Germany that served hours earlier as the backdrop for a show of unity by NATO leaders.

AP photographers saw other protesters storm a nearby Ibis hotel, setting fires and pilfering alcohol from its bar. An AP reporter saw intermittent scuffles between police and demonstrators in black as they gradually made their way back from the Europe Bridge and tried to enter the city center.

Around 2:45 p.m., the protesters — throwing rocks — tried to storm a massive police blockade at the Pont d'Anvers bridge, and were driven back by water cannon, tear gas, flash bombs and rubber bullets. Across the canal, nearly 1,000 people gathered to watch the fracas.

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Some of the protesters were hurt, but none of the injuries appeared to be serious.

Elsewhere, stacks of old tires were also set ablaze, unleashing thick plumes of black smoke that could be seen from across the river. Near the bonfire was a sign welcoming visitors to Strasbourg.

First lady Michelle Obama and other spouses canceled a visit to a cancer hospital out of concern for security, the French president's office said. Some 1,000 protesters were staked out near the hospital they were to visit.

Some of the protesters say they want an end to war and call NATO a tool of Western imperialism. Others simply appear bent on causing chaos.

Saturday's protest began calmly but began turning violent around noon (1000 GMT) at the same Pont d'Anvers bridge. An AP reporter saw police in body armor and helmets hoisting shields as they were pelted by several hundred protesters with rocks, sticks and then Molotov cocktails.

About 100 officers responded by lobbing flash bombs and volleys of tear gas into the crowds of demonstrators, many dressed in black and wearing masks or balaclavas.

Members of the violence-prone "black bloc" — named for their black clothes and hoods — then headed toward the Europe Bridge and set fire to the customs station on the French side and sprayed graffiti on the walls of buildings.

Later, German-run water cannon were driven across the Europe Bridge and used to help put out the fire. As the crowd dissipated, a convenience store was overrun and ransacked.

There were no immediate reports of injuries.

German Chancellor Angela Merkel and two dozen other NATO leaders walked across the bridge separating Germany and France before the 60th-anniversary summit began.

Across the Rhine River in Kehl, Germany, an estimated 5,000 demonstrators gathered peacefully and hoped to cross into Strasbourg but were diverted by scores of police, backed by at least five trucks with water cannons.

"No nations, no border. Fight law and order," they chanted in unison as police clad in riot gear looked on, before the demonstration began to gradually subside around 3 p.m. (1300 GMT).

Protesters have been frustrated by large police presence in both cities. Some 15,000 German police and 9,000 French police are on call for the summit. Helicopters have patrolled the skies and police in fast boats have crisscrossed the river, too.

Earlier Saturday, some 1,800 protesters left their camp south of Strasbourg at about 4 a.m. (0200 GMT) and headed north through deserted streets to the summit site before being turned back.

Elsewhere, a separate group of 200 French and German protesters — including dozens dressed as clowns and a team of dancing percussionists — occupied a central intersection in downtown Strasbourg after police launched flash bombs and tear gas at them.

German authorities had estimated that up to 25,000 protesters would take part in several demonstrations in Kehl and the German spa town of Baden-Baden, about 35 miles (55 kilometers) away, where NATO leaders met and had supper in a lavish casino.

France's interior minister has suggested 30,000 to 40,000 ultimately could show up in Strasbourg. The numbers have so far appeared much smaller.