'There was a massive attack with stones and bottles thrown,' said police spokesman Martin Otter. 'There were also injuries on both sides, but we don't have any details on numbers at this point. Stones and bottles are still being thrown.'

The attacks from about 400 militants on police accompanying a protest of about 5,000 leftists in the central Berlin district of Kreuzberg, a traditional hotbed of anarchist violence on Labour Day, was unusually fierce and began before darkness fell.

Authorities were bracing for violence with tension running high because of the economic crisis and rising unemployment.

There were clashes with police in the early hours of Friday in Berlin and in Hamburg. Several demonstrations turned violent across the country, with outbreaks reported in Ulm, Dortmund, Mainz and Verden.

Berlin remained tense and police expected further violence by the leftists, who were marching under a banner that read: 'Capitalism means war and crisis.' The militants destroyed a bus station, a cafe and damaged cars in an outbreak of violence that began shortly before 9pm 1900 GMT, 3am Singapore time).

Police pursued a general policy of 'de-escalation' but swooped in en masse to remove those spotted throwing stones.

Earlier, violence flared in the southern city of Ulm when left-wing militants threw bottles and stones at a march of about 1,000 neo-Nazis after police were unable to keep them apart.

Ulm officials said 29 riot police were injured. One female police officer suffered serious injures, police said, while 21 demonstrators were hurt.

One group of protesters in central Berlin pelted the German Finance Ministry building with brightly coloured paint bombs. -- REUTERS