Thousands of people are being stopped and searched by the police under their counter-­terrorism powers – simply to ­provide a racial balance in official statistics, the government's official anti-terror law watchdog has revealed.

Lord Carlile said in his annual report that he had "ample anecdotal evidence" of it happening, adding that such a practice was "totally wrong" and constituted an invasion of civil liberties.

"I can well understand the concerns of the police that they should be free from allegations of prejudice," he said. "But it is not a good use of precious resources if they waste them on self-evidently unmerited searches."

He said there was little or no evidence that the use of section 44 stop and search powers by the police could prevent an act of terrorism.

"While arrests for other crime have ­followed searches under the section, none of the many thousands of searches has ever resulted in a conviction for a terrorism offence. Its utility has been questioned publicly and privately by senior Metropolitan police staff with wide experience of terrorism policing," said Carlile. He added that such searches were stopping between 8,000-10,000 people a month.

Under the Terrorism Act 2000, the ­"section 44 stops" allow the police to search anyone in a designated area without suspicion that an offence has occurred.

But Carlile is critical of the use of the powers by the Met police, saying that he felt "a sense of frustration" the force did not limit its use of section 44 authorisations to some boroughs or parts of boroughs but used them across its entire area.

"I cannot see a justification for the whole of the Greater London area being covered permanently. The intention of the section was not to place London under permanent special search powers." He noted that the damage done to community relations was "undoubtedly considerable".

Examples of poor use of section 44 abounded. "I have evidence of cases where the person stopped is so obviously far from any known terrorism profile that, realistically, there is not the slightest ­possibility of him/her being a terrorist, and no other feature to justify the stop."

He later said that while the police should not discriminate racially, it was equally important that they should not balance the statistics.

"If, for example, 50 blonde women are stopped who fall nowhere near any intelligence-led terrorism profile, it's a gross invasion of the civil liberties of those 50 blonde women.

"The police are perfectly entitled to stop people who fall within a terrorism profile even if it creates a racial imbalance, as long as it is not racist."

Former British diplomat Sir Edward Clay told BBC Radio 4's The World Tonight programme he was subjected to a stop and search five weeks ago while on his way to work at the National School of Government, near Victoria Station in central London.

He said he had found the experience "sinister" and "intimidating". He told the programme: "I'm 63, I'm a grey-to-brown-haired white male, I'm 5ft 10 ins tall, looking extremely conventional."

The latest police figures show that ­117,278 people were stopped under section 44 in 2007-08, of whom 73,967 were white, 20,768 were Asian and 15,218 black.

The Met has announced a review of how it uses section 44 powers. The home secretary, Alan Johnson, is to issue fresh guidance to the police, warning that counter-terrorism must not be used to stop people taking photographs of on-duty officers.

Carlile endorses complaints from professional and amateur photographers that counter-terror powers are being used to threaten them with prosecution if they take photographs of officers. He said the power was only intended to cover images likely to be of use to a terrorist.

The police had to come to terms with the increased scrutiny of their activities by the public, afforded by equipment such as video-enabled mobile phones. "Police officers who use force or threaten force in this context run the real risk of being prosecuted themselves for possible criminal and disciplinary offences," he warned.

The Association of Chief Police Officers (Acpo) said new guidance on stop and search had been issued. Acpo counter-terror spokesman Assistant Commissioner John Yates said: "Powers to stop and search are an important part of tackling terrorism but should always be used in a proportionate way by well-trained and briefed officers. Used correctly, stop and search is a powerful tool that can help protect all our communities from terrorism.

"We have been fully engaged with Lord Carlisle's review and chief officers will study the report carefully. The threat to London from terrorism is real and these powers are a vital tactic in our counter-terrorism strategy."