The exercise resembled scenes from the suicide bombing that hit public transport in July 2005 [GETTY]

British police, fire and ambulance staff held a huge pre-Olympics security exercise on Wednesday centring on a mock terrorist attack on the London subway system.

The two-day test - called "Exercise Forward Defensive'' - started at the Aldwych subway station, which has been closed to commuters since 1994. The London Underground maintains the station so it can be used in films and rented for parties.

The security simulates an attack on one of the busiest days during the 2012 London Olympics.

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Authorities declined to reveal the exact scenario in advance, saying surprise is a key element of the exercise.

"(It's about ensuring) that we have the right people in the right places, that we understand how others operate and that we are talking to each other at the right levels and in the right way,'' said Assistant Commissioner Chris Allison of the Metropolitan Police.

The test is a part of efforts to create confidence ahead of the games, which open July 27 and end August 12.

British Transport Police spokesman Simon Lubin said for participants the test evoked memories of the July 7, 2005, London transit attacks, when four suicide bombers killed 52 commuters aboard three tube trains and a bus.

The attacks came a day after London was awarded the 2012 Olympics.

Official reports and an inquest criticised the emergency services' response to the 2005 bombings.

"If there are mistakes, this is the time to make them, not when there's a real incident,'' Lubin said.