STRASBOURG, France – Riot police used tear gas and rubber bullets to beat back a crowd of several hundred anti-NATO protesters in Strasbourg on Thursday as the city in eastern France, host of the alliance's 60th anniversary summit, readied for the arrival of world leaders.

Police lobbed hundreds of tear gas canisters during clashes between about 40 riot police and some 200 masked protesters in the neighborhood of Neuhoff, in the southern part of the city near the German border.

The masked protesters had been among a crowd of about 800 demonstrators when they broke away and repeatedly pelted officers with rocks and bottles. There were no immediate reports of injuries. Police said a dozen protesters were arrested. At one point, protesters set fire in the middle of a street to building materials they had scooped up from a nearby construction site.

By late Thursday, calm had returned to Strasbourg, but two more full days of planned protests lie ahead.

Tens of thousands of demonstrators have descended on Strasbourg and two southwest German towns to protest the cross-border summit of members of the North Atlantic Treaty Organization, which begins Friday. In Strasbourg, many shops and small businesses have shut down during the summit.

Twenty-eight world leaders will attend the two-day summit, including President Barack Obama, French President Nicolas Sarkozy and German Chancellor Angela Merkel.

In a bid to prevent violence, France has temporarily reinstated border controls with its immediate neighbors for the meeting.

As demonstrators gathered in Strasbourg, one Colombian activist, Gregorio Yong, said NATO was "a synonym of war." Daniel Mouray, a protester wrapped in a rainbow flag with "peace," said he expected little from the summit.

"We want the wars to stop and for the Americans to go back home," he said.

German authorities estimate that up to 25,000 protesters will take part in several demonstrations in the German cities of Baden-Baden and Kehl, while France's interior minister has suggested 30,000 to 40,000 could show up in Strasbourg, where a camp has been set up to house demonstrators.

German and French police have said 2,000 to 3,000 members of the violence-prone "black block" — so-called for the black clothes and hoods they wear — were expected.

Some 15,000 German police — including 31 riot squads — and 9,000 French police will be on hand for the summit.

Protest organizers have called for peaceful demonstrations to highlight their complaints, including anti-war, anti-globalization, anti-capitalist and disarmament platforms.

The German chapter of the left-wing Attac group, which is calling for the withdrawal of all NATO troops from Afghanistan and an end to the military alliance, climate protection and a "just world economy," was organizing conferences, camps, demonstrations and blockades.

"The world leaders will be confronted with our protest and our call for a peaceful world," said Attac spokeswoman Gudrun Reiss. "We're calling for massive participation."

The Berlin Anti-fascist Left group said it was sending at least 10 busloads of demonstrators.

After an evening protest Thursday in Baden-Baden, the main demonstration begins Friday at noon in Kehl. The activists plan to march across the Rhine river into Strasbourg, with another protest planned for Saturday.

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David Rising reported from Baden-Baden, Germany