Security services tracked an alleged British al-Qaida terror cell for months in a major covert operation, the Old Bailey heard.

Listening devices were placed in cars and houses belonging to two of the suspects and extensive CCTV footage was taken of the gang's movements.

The court heard how thousands of hours of audio tape were recorded, broken down into 60 transcripts, and then pored over by police officers.

The surveillance operation swung into action in late January 2004 and ran until the arrest of the defendants on March 30 of that year.

The seven defendants are accused of conspiring with a Canadian to cause explosions in Britain.

Among them are Omar Khyam, 24, his brother Shujah Mahmood, 19, Waheed Mahmood, 34, and Jawad Akbar, 22, all from Crawley, who deny the charge.

Salahuddin Amin, 31, from Luton, Bedfordshire, Anthony Garcia, 23, from Ilford, east London, and Nabeel Hussain, 20, from Horley, Surrey, deny the same charge.

Khyam, Garcia and Hussain deny a charge under the Terrorism Act of possessing 1,300lb (600kg) of ammonium nitrate fertiliser for terrorism. Khyam and Shujah Mahmood also deny possessing aluminium powder for terrorism.

It is alleged they talked about bombing the Bluewater shopping centre in Kent, a London nightclub and trains, as well as poisoning football fans with spiked beer.

Junior prosecution counsel Mark Heywood told the jury yesterday that the covert operation, codenamed Crevice, was divided into three segments observation by officers, bugging and CCTV surveillance.

It was a joint operation by the security services, Special Branch and the Metropolitan Police's anti-terrorism squad.

They bugged an address in Slough, Berkshire, where Khyam was staying and Akbar's home in Uxbridge, west London.

Listening devices were placed in Khyam and Garcia's cars.

Another device was placed in the Access Self Storage depot in Boston Road, Hanwell, west London, where the plotters allegedly stored the fertiliser.

Surveillance cameras were placed in seven locations, including the self-storage depot, the two suspects' addresses and three internet cafs.

The jury was shown CCTV footage filmed in Slough and Crawley.

The trial continues.