Six men arrested under terror laws during the papal visit last year were never involved in any plot to kill the Pope, but police used their powers "lawfully and appropriately", a review has found.
David Anderson QC, the independent reviewer of terrorism legislation, said: "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."
But he added that the police used their powers under the Terrorism Act 2000 "lawfully and appropriately".
But Mr Anderson warned that there will be "future temptations" to use the anti-terrorism powers "in relation to individuals as to whom the necessary reasonable suspicions do not exist", particularly as the UK prepares to host the Olympic Games next year.
"Constant vigilance is required to ensure that the legal boundaries of those powers are respected, as they were in this case," he said.
The six men, who were all Muslims of North African origin, were released without charge after being arrested in London last September.
The Government's terrorism watchdog launched the review last November to examine whether the police used their powers correctly when they arrested the men and whether there was any other way they could have dealt with the suspected threat.
Searches of the premises did not uncover any weapons or suspicious materials, Scotland Yard said. Reports at the time suggested the men were all street cleaners and had simply been overheard sharing a joke in their canteen.
Home Secretary Theresa May said: "I am grateful to David Anderson for his detailed report - his first as independent reviewer for terrorism legislation.
"I am also 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. I welcome both his finding and his recommendations and intend to publish the Government's full response shortly."