Around 10,000 anti-capitalist protesters clashed with police and at one stage blocked all roads to a resort in northern Germany where world leaders were gathering today for the G8 summit.
Police turned water cannons on some 2,500 protesters in order to clear one of the roads to the Group of Eight summit in Heiligendamm. Police detained some 160 protesters in the skirmishes.
Delegates from several G8 countries, including host Germany, said the protests were limiting their ability to move around at the summit venue, a seaside resort on Germany's Baltic coast.
Eight officers were injured during earlier clashes with protesters near the town of Bad Doberan, police spokesman Luedger Behrens said. Police "used water cannons twice after demonstrators bombarded police with stones," he said.
The demonstrators were trying to block access to a luxury hotel on the coast in Heiligendamm where G8 leaders including US President George W. Bush were gathering.
By late afternoon, all three roads leading into Heiligendamm were blocked off by protesters before police moved in to clear one of the roads.
One German official said the only way into Heiligendamm had been via boat or helicopter. Another said the demonstrations had become a "major problem" for journalists covering the summit.
"We're stuck here now. The whole place looks shut down," an official from one of the G8 delegations said by telephone from inside the summit venue.
The protesters themselves were in a jubilant mood.
"It's a great success just to be able to sit here," said Gunar Finke, a student from the southern German city of Freiburg. "It's only a symbolic step but it's an important one to show we're against G8 policies."
"We had a super plan and we surprised the police, who didn't know how to stop us," he said.
Some 16,000 security personnel are in the area. World leaders are shielded from thousands of demonstrators by a 12-kilometre fence topped with barbed wire.
The Federal Constitutional Court, Germany's highest court, ruled against plans for a demonstration on Thursday outside the fence. But with some protesters already at the fence that decision had been made largely irrelevant.