* Worst May Day violence in Berlin for four years

* Injuries on both sides, say police

* Tension aggravated by economic crisis

(Updates Berlin violence, police injured in Ulm)

By Erik Kirschbaum and Brian Rohan

BERLIN, May 1 (Reuters) - Left-wing militants pelted riot police with stones, bottles and firecrackers in Berlin on Friday evening in the most severe May Day violence to hit the German capital in four years.

"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 9 p.m. (1900 GMT).

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 demontrators were hurt.

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

BANK WINDOW SMASHED

Before dawn about 200 demonstrators chanting anti-capitalism slogans threw stones at police in Berlin, injuring 48 officers. In Hamburg, a bank window was smashed.

"There are people out in the streets protesting peacefully against the economic crisis and there's nothing at all wrong that," said Berlin police spokesman Frank Millert.

"But when people burn cars and trash containers and commit other criminal acts -- that has nothing to do with political protests," he told N-TV television.

May Day is traditionally marked by union rallies in many European countries but the global economic downturn led to significantly larger crowds in Germany this year. Union leaders estimated 484,000 took part in 400 rallies throughout Germany.

The country is facing its worst recession since World War Two with the economy forecast to contract by 6 percent in 2009.

"The crisis that is dominating us was caused by unscrupulous speculators," said Michael Sommer, head of the DGB Labour Federation in a speech in the western city of Bremen.

"This crisis is worse than anyone expected. There is no light at the end the tunnel."

Rioting on the May 1 Labour Day holiday had been on the wane in the past three years after police shifted tactics from battling rioters to de-escalation. However, the economic crisis has aggravated public anger at the growing disparity in wages.

Frank-Walter Steinmeier, who is leading the Social Democrats into the September election against Chancellor Angela Merkel's Christian Democrats, said he understood workers' rage.

"The crisis is a result of greed and irresponsible actions," he told a rally in the western town of Ludwigshafen. (Writing by Erik Kirschbaum; Editing by Andrew Dobbie)

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