Rocks, bottles and sticks were thrown at riot police, who used tear gas and batons to try to disperse the crowd.

A police spokeswoman described it as a "massive assault", but it only involved a small portion of the estimated 30,000 people who had joined the rally.

Up to 100,000 anti-globalisation demonstrators from more than 160 groups gathererd last night for a march and rally.

They were there to protest against next week's G8 summit of leading industrial nations in Heiligendamm, 16 miles away.

German authorities said there were serious concerns that far-left groups are planning violent attacks.

At least 13,000 police were on duty in Rostock, where the authorities said they would react quickly if there was trouble.

Protesters marched into central Rostock before stopping for a rally and a concert in the late afternoon.

Many began yesterday at a camp by the harbour in Rostock, where the main entrance bears the words: "No police and no neo-Nazis."

The activists pitched their tents in a large field and the majority are planning to stay until next week's G8 summit.

"The world shaped by the dominance of the G8 is a world of war, hunger, social divisions, environmental destruction and barriers against migrants and refugees, " read a message printed on leaflets handed out on the streets of Rostock.

"We want to protest against this and show the alternatives."

Many protesters travelled to Rostock from all over Europe.

Some activists made it by bicycle from Denmark, while others arrived by train or bus.

Most protesters said they want to stage a peaceful demonstration to vent their anger at G8 leaders. However, some shopkeepers were taking no chances, with many in the city boarding up their windows as a precautionary measure.

"It's very important people all over the world come and protest against the politics of G8, which actually refer to all people of the world, although they are driven from the high elite in the G8 countries, " said one protester.

"They actually use citizens of the world and the environment of the world as their playground to achieve more money and power."