(Updates with arrest, changes dateline from Islamabad)

LONDON, Sept 7 (Reuters) - A British man held in Pakistan without charge for more than a year on suspicion of links to al Qaeda was arrested immediately on returning to Britain on Friday.

Nine armed, uniformed police officers and two detectives were waiting for Rangzieb Ahmed as he flew into London's Heathrow airport on a commercial flight from Islamabad.

As soon as the doors were open, police boarded the plane and minutes later escorted Ahmed from it in handcuffs and put him in the back of a police van.

A Greater Manchester police spokeswoman said the 32-year-old was being taken to the northwestern city to be questioned in connection with a terrorism investigation in which a man and a woman had been charged. She declined to give further details.

According to international monitoring group Human Rights Watch (HRW), Ahmed was arrested in August last year in Pakistan's North West Frontier Province. Pakistan's Supreme Court ordered his release on Aug. 31.

The rights group quoted Ahmed as saying in an interview provided by intermediaries he had been repeatedly tortured by Pakistani security agencies and questioned by U.S law enforcement personnel and British security services.

Reacting to his arrest, HRW's South Asian researcher Ali Dayan Hasan said: "We suspected this would happen. The question is why. If there is proof against him, why has he not been charged in Pakistan?"

He said Britain should have requested Ahmed's extradition if it had a case against him, instead of waiting for the Pakistan court to declare his detention illegal.

He cited the case as an example of the "cloak and dagger" way that the war on terrorism was being fought.

HRW said Ahmed had been involved with militant groups fighting Indian security forces in the disputed Himalayan region of Kashmir and jailed in India. British authorities had secured his release from India and he had returned to Britain before coming to Pakistan in 2006.

"He denies any subsequent involvement in militancy or any connection with the al Qaeda network," it said.

Several British militants have visited Pakistan, including two of the four suicide bombers who killed 52 people in London in July 2005. Rashid Rauf, a Pakistani-British man suspected of involvement in a plot broken up in Britain last year to blow up U.S.-bound airliners, is also in custody in Pakistan.

A Pakistani rights worker also said Ahmed had been tortured.

"He was tortured in custody for one year without any charge by American, British and Pakistani intelligence agents," said Khalid Khawaja, a former official of Pakistan's Inter-Services Intelligence agency who later became a rights activist.

AlertNet news is provided by