The British Royal Marines have joined police on the Thames in London on Monday for a week-long security exercise in preparation for the 2012 Olympics.

The Green Berets have joined the Metropolitan Police's Marine Policing Unit for a series of training exercises on the Thames, which involve more than a dozen craft on the water, plus helicopters, the Ministry of Defense said.

More than 140 commandos and police personnel will be involved in the five-day exercise which is designed to ensure the smooth running of the security mission during the Olympics this July and August.

The Metropolitan Police's Assistant Commissioner Chris Allison said it is important that civilians and the military can work hand-in-hand during the large-scale operation, and all involved understand the Thames and understand each other's tactics.

"The Thames runs through the very heart of our capital and will be a popular place for people who want to be part of the Olympic spirit," he said.

Allison, who is also the National Olympic Security Coordinator, said the week-long police-marines exercise was "not in response to any specific threat", rather "part of our planning to pre-deploy certain specialist assets to bolster our operation."

As well as the Royal Marines' presence on the Thames, Britain's biggest warship, HMS Ocean, will also be involved in the Olympics security mission this summer, mooring on the river at Greenwich as a base for personnel and for helicopters, including the Fleet Air Arm's Lynx.