Small mobile groups of protestors played cat and mouse with battalions of French riot police on Saturday, seeking to cause maximum disruption on the final day of a Nato summit in Strasbourg.

As the Atlantic alliance's 28 leaders, including US President Barack Obama, gathered for talks, contingents of up to 200 demonstrators penetrated the outer security cordon and succeeded in blocking traffic in several places.

There was a heavy police presence, with 10 000 CRS riot police deployed in buses around Strasbourg, armed with batons, helmets and body armour. They were backed up by water cannon and helicopters buzzing overhead.

Police fired tear gas to stop around 1000 masked protestors waving flags and banners from crossing a bridge into central Strasbourg, but march organisers promised that more would arrive for a "monster" demonstration later.

Meanwhile, a Franco-German river patrol arrested around a dozen divers from the environmental group Greenpeace who attempted to enter the Rhine in wetsuits and swim toward a bridge hosting a photo opportunity for the leaders.

Police and protestors both claimed early victories Saturday but it was set to be a long day, with organisers hoping as many as 60,000 demonstrators would proceed along a route approved by police for the main march.

"We are winning the war," a officer from the CRS said at the bridge where the marchers were pushed back.

But a 45-year-old German demonstrator whose group blocked a tram route taking journalists to the summit venue disagreed.

"We are near the congress centre ... It is a first victory for us because President (Nicolas) Sarkozy said that he didn't want to see demonstrators on the street," Martin, who declined to give his surname, told AFP.

"We are on the street, there are thousands of others on the streets."

"The police are attacking us from the first moment with tear gas and stun grenades, they are trying to shoot us with rubber bullets. But we are calm and we said we want to make our civil disobedience," he said.

"Nato is a coalition of war — think about Afghanistan, think about Iraq, think about the new strategy for global security systems, think about the war on terror," he said.

Many of the demonstrators were masked and wore black, brandishing red and black flags, peace banners, beating drums and carrying pictures of Cuban leader Che Guevara. Some had gas masks.