A group of peace protesters today won their High Court battle over a police decision to detain them en route to a demonstration against the war in Iraq.

Three coaches carrying the protestors were stopped from attending a vigil at RAF Fairford in Gloucestershire in March last year.

Around 60 of the 120 passengers took legal action against Gloucestershire Police, arguing that their "unjustified" detention amounted to an abuse of power.

Lord Justice May and Mr Justice Harrison, sitting in the High Court in London, today ruled that their detention and forced return to London could not be justified under common law and the European Convention on Human Rights.

The protesters failed to gain a ruling that they should have been allowed to attend the demonstration however.

The judges agreed that, even though their subsequent detention was unlawful, the police had been entitled to stop them reaching Fairford because of fears of a breach of the peace.

The coaches from London were intercepted in a lay-by in Lechlade at 12.50pm, searched and items were seized. At 2.15pm, police decided to escort the coaches non-stop back to London to prevent violence by hard-core demonstrators.

Michael Fordham, appearing for demonstrator Jane Laporte, from Tottenham, London, argued that both the action of turning away and of forcible return were unlawful.

Lord Justice May ruled the police were entitled to take preventive measures to avoid breaches of the peace. He added it had been impractical for officers to deal with a large number of "uncooperative" people on an individual basis in the lay-by.

The judge said: "I do not consider that the police action in preventing the coaches from proceeding to Fairford was unlawful."

But he continued to say that the detention of the passengers while they were escorted back to London was a breach of their right to liberty under Article 5 of the human rights convention.

People detained to prevent a breach of the peace should be released unconditionally "as soon as the immediate apprehension of breach of the peace is past". How long transitory detention without arrest could lawfully last depended on the facts of the individual case – "but it cannot be for long", the judge said.

The judge rule that Ms Laporte's enforced return to London on the coach was not lawful because "there was no immediately apprehended breach of the peace by her sufficient to justify even transitory detention".

The judge added that the "circumstances and length of detention on the coach were wholly disproportionate to the apprehended breach of the peace."

The judge said he appreciated the court's ruling "may cause difficulties for the police in circumstances such as those at Fairford on March 22 20003".

The demonstrators and the police were given permission to appeal against those parts of the ruling that went against them, because of the importance of the issues raised. Any decision on damages the demonstrators could receive will await the outcome of the appeal.

During the recent High Court hearing, Michael Fordham said that the police operation at RAF Fairford, which was used by American B-52 bombers, was the largest and most complex ever undertaken by Gloucestershire Police.

The protesters were utterly opposed to the US-led military assault on Iraq and wished to exercise their deeply-held beliefs through peaceful protest.

Mr Fordham said that the police regarded their operation as a great success in achieving their strategic objectives of preventing violence and facilitating peaceful protest.

Lawyers for Gloucestershire Police argued that their officers were not only entitled to take the action they did – but that they were obliged to.