Berlin  - Police and demonstrators have been flexing their muscles in the run-up to this week's summit celebrating the 60th anniversary of the founding of NATO.

Around 150 anti-NATO campaigners clashed with police Tuesday night on the outskirts of the French city of Strasbourg, co-host to the April 3-4 gathering along with Kehl and Baden-Baden in Germany.

Police used tear gas to disperse the protesters who had gathered to voice their anger at the strict identification checks imposed to prevent troublemakers from disrupting the summit.

Across the Rhine river in Germany, four protesters were detained a day earlier when 2,500 turned up for an anti-NATO rally organized by left-wing groups in the city of Freiburg.

Security officials are taking no chances ahead of the mammoth event, which is expected to see the 28-member alliance unveil a new strategy for the war in Afghanistan.

Some 14,600 police will be guarding the 3,500 delegates on the German side, while 10,000 will be patrolling the streets of Strasbourg, where the main talks will be held.

German security officials said they expect up to 25,000 demonstrators to turn up in Germany, around 3,000 of whom have been classified as potentially violent.

Police in Strasbourg believe about 2,000 militants could be mingling among the demonstrators on French soil and fear the city will be worse affected than the venues in Germany.

"We've got the camp where anti-NATO campaigners are staying and are also host to the assembly of the NATO leaders. The Germans don't have that," a police spokesman said.

Protesters have registered 13 separate demonstrations in the two summit locations in Germany. They have also threatened to block roads and carry out other acts of civil disobedience.

Germany is providing 600 soldiers to help protect the summit, while NATO will deploy AWACS surveillance aircraft. Shipping along the Rhine will be halted on Saturday morning when the heads of state and government pose for photographs on a bridge linking Strasbourg to the German side.

In Baden-Baden, police have erected security zones around key venues, including the hotel where Chancellor Angela Merkel is staying and the building where the leaders will hold their opening dinner on Friday evening.

Manhole covers have have been welded shut and parking metres removed from some city centre locations as a precaution against terrorist attacks with explosives.

People in flats overlooking the town square have been told to keep their windows closed and refrain from using their balconies when Merkel takes a stroll there with US President Barack Obama on Friday afternoon.

Not everyone is happy with the tight security arrangements.

Marianne Wasserburger complained that she was forced to cancel a function in her bookshop overlooking the town square because of the Obama visit.

"The shop is open, but no customers will be able to enter because of the security barriers. I can't even look out of the window to see what's going on." (dpa)