Around 25,000 troops and police officers will be deployed in London during the Olympic Games to prevent attacks, a Scotland Yard official told AFP on Thursday.

"The whole purpose of our policing operation is to try and deter terrorists, to deter those wish to do ill to the Games," Assistant Commissioner Chris Allison said during a security demonstration on the River Thames.

Around 100 Royal Marines and 50 police officers rehearsed a series of drills in patrol boats as a Lynx Navy helicopter flew ahead.

The drills, which the Metropolitan Police said would continue for a week, are aimed at familiarising troops and police ahead of the summer -- when Britain will launch its biggest ever peacetime security operation.

Major Paul Stitson of the Royal Marines said the police and military were working closely together.

"We have been developing our local knowledge and assessing threats," he said. "We always prepare for the unknown."

London's 553-million pound ($856-million, 664-million-euro) security operation for the Games will also involve warplanes, two navy ships and surface-to-air missiles.