Strasbourg, France  - Rioters set fire to a five-storey hotel and two low-rise buildings in Strasbourg Saturday as left-wing protests against a NATO summit in the French city turned violent.

French security sources said some of the rioters, believed to be mainly anarchists dedicated to bringing down the western system, had been carrying loaded guns when police arrested them.

The hotel fire, apparently caused by petrol bombs, spread through the ground floor of the building, which had been vacated because of the summit, and then took hold up to the roof. Fire also gutted a nearby tourist information office and a disused French customs post.

It was not immediately known if there were any casualties.

One French police officer claimed the hotel was attacked in the belief that it was being used to accommodate out-of-town police.

"It's no coincidence," he said bitterly. Police spokesmen declined to say if officers had been staying in the 78-room hotel.

Hundreds of police protected French firefighters as they fought the blaze. French riot police, armed with clubs and shields, repeatedly fired tear-gas grenades to drive off rioters.

All the fires occurred near a traffic bridge spanning the Rhine River as 6,000 German protesters waited on the other side to cross and join the main anti-summit demonstration planned for the afternoon. German police blocked the bridge because of the mayhem.

Angry protesters sat down, complaining their rights had been infringed, but calmed after German police proposed another route into Strasbourg. The violence on the French side later ebbed with militants melting away into the city.

European pacifists, anarchists, communists and radical feminists from several nations had formed a protest alliance before the summit.

Many were avowedly non-violent, but the movement also embraced a hardline group that has repeatedly clashed with police at summits and other government events in European nations down the years.

NATO leaders were meeting in Strasbourg on Saturday at a summit celebrating the 60th anniversary of the alliance's founding.

Sporadic clashes Thursday evening and Friday evening had given a foretaste of Saturday's attack by the militants, many of them carrying stones, empty bottles and petrol bombs to throw.

The militants wore black clothing as a kind of uniform and ski masks to conceal their faces from police video cameras.

Some 10,000 French police officers and gendarmes, reinforced by some German police units, were deployed in and around Strasbourg.

Police said 28 people were arrested early Saturday as nearly 2,000 demonstrators managed to infiltrate the city, blocking tramway and bus routes. But they failed in their primary goal of blocking the motorcades carrying NATO leaders through Strasbourg.

The demonstrators' motto was a call to spend "millions on peace instead of billions on war." (dpa)