Campaigners have won a legal battle to prove their rights to protest were violated when police stopped them from attending an anti-war demonstration.
Police say they were protecting protesters' rights
About 120 Iraq war protesters were held on coaches by police near RAF Fairford in Gloucestershire in March 2003.
The High Court and Court of Appeal had already ruled police acted unlawfully in holding protesters on the coaches.
But on Wednesday Law Lords ruled police did violate the right to freedom of expression and lawful assembly.
This overturned a previous High Court ruling that police did not violate the protesters' rights in this respect.
Five Law Lords also dismissed a cross-appeal by the police against the "unlawful" decision to hold protesters on the coaches.
Peace campaigner Jane Laporte, under whose name the case was brought, said: "I am absolutely overjoyed.
"The Lords have confirmed that freedom to protest is something that should be treasured in this country and police don't have the right to take it away."
Gloucestershire Police said it was "disappointed" with the decision, which it accepted, and added that officers acted in "good faith".
It also expressed "regret" for any inconvenience and said it would now review its policies.
"Policing in [such] scenarios is difficult and complex, with competing rights and responsibilities having to be assessed and acted upon in real time by operational commanders," said a spokesman.
"Intelligence pointed to the potential for further disorder at the base and it was against this background that the decision was made to stop and turn back the three coaches travelling to RAF Fairford from London."
The buses did not stop during the journey back to London
Lord Bingham said: "It was entirely reasonable to suppose that some of those on board the coaches might wish to cause damage and injury to the base, and to enter the base with a view to causing further damage and injury.
"It was not reasonable to suppose that even these passengers simply wanted a violent confrontation with the police, which they could have had in the lay-by.
"Nor was it reasonable to anticipate an outburst of disorder on arrival of these passengers in the assembly area or during the procession to the base."
The Lords ruled the police actions were unlawful because they were not prescribed by law and were disproportionate.
Alex Gask, legal officer for human rights group Liberty, which had intervened as an interested party in the appeal, said: "Nothing less than our freedom of speech was at stake in this case.
"Unmerited concerns (on the part of the police) about some future breach of the peace cannot justify the denial of this fundamental right."
Ben Emmerson QC, representing the protesters, told the Law Lords that it was a fundamental right in Britain for citizens to gather to demonstrate peacefully on matters of public interest.
He said it was the responsibility of the police to maintain public order "in a manner which fully respects the rights of those who wish to demonstrate peacefully".
Police who authorised two coach-loads of protesters to be stopped and passengers searched while being detained - and then escorted back to London - had breached that right, he said.
Police lawyers had argued that rather than interfering with passengers' human rights, they were upholding them by protecting their lives, which would have been put at risk if they had broken into the air base.