Police warned not to misuse anti-terror laws to round up innocent people

Police must avoid the "temptation" to abuse anti-terrorism laws by using them as a net to round up innocent people – particularly in the run-up to next year's Olympics, the Government's terrorism watchdog has warned.

David Anderson QC, the independent reviewer of terrorism legislation, cautioned police while ruling yesterday that six men arrested during the papal visit last year were never involved in any plot to kill the Pope.

The half-dozen Westminster street cleaners, who were all Muslims of North African origin, were seized at gunpoint on the basis of a "barely credible" tip-off, Mr Anderson found.

The watchdog warned that there will be "future temptations" to use the anti-terrorism powers, acquired under the Terrorism Act 2000, "in relation to individuals as to whom the necessary reasonable suspicions do not exist".

Mr Anderson expressed particular concern "in the context of international high-profile events such as the London Olympics". "Constant vigilance is required to ensure that the legal boundaries of those powers are respected, as they were in this case," Mr Anderson concluded. Scotland Yard counter-terrorism officers launched "Operation Grid" after a tip-off that the six men, who worked for Veolia Environmental Services, had been seen looking at a picture of the Popemobile in a newspaper.

The source said the men had expressed a desire to kill the Pope as an act of revenge for the recent burning of a Qu'ran and that "there were virgins waiting for them" if the attack succeeded.

The men, aged 26 to 44, who deny any such conversation took place, were arrested at gunpoint and questioned at the high-security Paddington Green police station on the second day of the papal visit. Police searched eight homes in north and east London and two business premises in central London, including a street cleaning depot, as part of the investigation.

Searches of the premises did not uncover any weapons or suspicious materials, Scotland Yard said. They were released without charge, having been held for between 33 and 42 hours. All street cleaners were removed from the Pope's London route as a precaution, the report found.

The watchdog found that police had reason to be suspicious – they had used their powers "lawfully and appropriately" in this instance, he said, because of the fact that 10 street cleaners' uniforms were missing. But there were other reasons for Scotland Yard to react sceptically to the initial tip-off.

It did not conform to any established pattern of intelligence, it was "barely credible" that those involved in such a plot would talk about it in public, and the conversation was only on a general level.

Their arrest was prompted by a canteen joke that had been misunderstood, it later emerged.

"There is no reason to believe, with the benefit of hindsight, that any of the arrested men was involved in a plot to kill the Pope, or indeed that any such plot existed," Mr Anderson found.

Mr Anderson said he was "in no doubt that to be subjected to an armed arrest at their place of work was a shocking and disorientating experience" for the suspects. "Whilst none of the men complained of any mistreatment during periods of detention, this too must have been a bewildering and unpleasant experience for them," he said.

The watchdog called for police forces to review their procedures for enabling suspects to let someone know about their arrest. Even when given the chance to call someone, at least two of the suspects were unable to do so because the numbers were stored on their mobile phones which had been seized and they could not remember them.

Theresa May, the Home Secretary, welcomed Mr Anderson's report and promised to publish a full Government response to his recommendations.

She said she was "pleased that he finds that the police exercised the powers afforded them under the Terrorism Act 2000 lawfully and appropriately in seeking to prevent what they had reasonably suspected was a potential terrorist plot".