Hamburg/Berlin - Leftist radicals rioted in three German cities overnight Thursday and into Friday, clashing with riot police who had anticipated trouble in the traditional May Day parades.

The clashes tended to centre on opposing rallies by the far-right National Democratic Party (NPD).

In the worst violence in the northern port city of Hamburg, police said they detained 250 rioters.

The leftists set fire to a police car, six private cars and 3,000 tyres in a storage yard. They set bonfires on the streets and in a park in a bid to draw off police and force cancellation of the parade by 1,500 right-wing marchers.

After an initially quiet start to events in Berlin, leftists attacked police in the Kreuzberg part of the city, targeting Berlin Police Chief Dieter Glietsch with rocks and bottles and forcing him to seek shelter in a protected police vehicle.

Kreuzberg has for years been the scene of violent clashes on the May Day holiday between leftist radicals and the police.

In Hamburg, there were scuffles between rightist and leftist activists.

The NPD parade went ahead, hours late, after police cleared a path through the crowd using powerful water jets mounted on tanker trucks.

Police counted 6,600 demonstrators opposed to the anti-immigrant NPD, which has claimed to be a voice of the German poor and demanded to hold a labour day parade in an old-time working-class district of Hamburg.

The 2,500 police officers struggling to separate the two groups were pelted with stones and firecrackers, and 20 were injured.

After nightfall, the leftists, who say they are acting to end imperialism and oppression in an annual May Day ritual, withdrew to their home base in the city and skirmished with police for a second night running.

In Nuremberg, 1,500 NPD supporters paraded through empty streets but were pelted with eggs and bottles during a final rally, as some of nearly 10,000 anti-NPD demonstrators managed to get within throwing range after scuffles with police lines. Four police officers were injured.

Many Germans were outraged that the NPD had marched in Nuremberg, the city adopted by dictator Adolf Hitler as the home of his Nazi Party. Attempts to proscribe the NPD because of its alleged pro-Nazi views have never succeeded.

The NPD, believed to have 7,000 card-carrying members, has seats in two of Germany's 16 state parliaments but none at the federal level.

Police said they were obliged to provide the marches with security cordons in the name of free speech and to avert serious bloodshed.

At a safe distance from the violence, mainstream labour and religious groups held peaceful rallies in both Hamburg and Nuremberg to denounce the NPD. German Social Democrats (SPD) and labour leaders called Saturday for renewed efforts to outlaw the far-right party.

Bavarian State Premier Guenther Beckstein, who comes from Nuremberg, said that his government would use undercover agents, court challenges and youth education programmes to undercut the NPD wherever possible.

The violence was associated with the Black Block, a loose movement of several thousand German anarchists who led violent demonstrations in June 2007 against the Group of Eight (G8) summit in Heiligendamm, Germany, and who seek to physically confront the NPD. (dpa)