London A controversial police operation to "kettle" protesters in Oxford Circus has been judged lawful by the European court of human rights, which has ruled the containment was the "least intrusive and most effective" tactic available to officers.

The ruling by the Strasbourg court is the latest in a series of legal setbacks for campaigners, who over the last decade have repeatedly failed to challenge the power of police to contain demonstrators.

The case has taken more than a decade to reach the court in Strasbourg and was the most significant among a host of legal challenges to the crowd control technique.

Seventeen judges at the grand chamber of the court, which hears cases deemed the most important, considered a case brought by one demonstrator and four bystanders who were swept up in a police cordon at Oxford Circus in 2001.

They had claimed they were deprived of their right to liberty under article five of the European convention on human rights.

Any judgment in favour of the applicants, which was opposed by the UK government, would have required the Metropolitan police to fundamentally rethink public order tactics less than five months before the Olympics.

However, in a majority 14 to three vote that endorsed the decisions made by the Met in the case, the court held that an original 2005 high court finding in favour of the force still stood.

Violence from minority

"The court finds no reason to depart from the judge's [2005] conclusion that in the circumstances the imposition of an absolute cordon was the least intrusive and most effective means to be applied," the ruling said.

It added: "Moreover, again on the basis of the facts found by the trial judge, the court is unable to identify a moment when the measure changed from what was, at most, a restriction on freedom of movement, to a deprivation of liberty." The Strasbourg judges also noted how, after a cordon was placed around thousands of protesters at 2pm during the May Day protest, the Met repeatedly attempted to release people held within the pen.

But violence from a "significant minority" within the crowd prevented them from doing so.

"In these circumstances, where the police kept the situation constantly under close review, but where substantially the same dangerous conditions which necessitated the imposition of the cordon at 2pm continued to exist throughout the afternoon and early evening, the court does not consider that those within the cordon can be said to have been deprived of their liberty within the meaning of article five."

The ruling marks the probable end of a 10-year campaign by protester Lois Austin, one of the applicants in the case. Austin, now 42, was prevented from picking up her 11-month old daughter from a creche when she was held in the 2001 kettle. The other applicants, who were bystanders not connected to the protest, and applied to the court separately, were George Black, 62, Bronwyn Lowenthal, 40, and Peter O'Shea, 48.