Since the new era of anti-globalization/anti-capitalism protests hit Britain a decade ago, the police have used a method of crowd control called a "kettle." Basically protesters can go into an area, but then they cannot get out until the police deem the steam to have risen and dissipated through the spout. Or the police can allow a few out at a time.
I have been kettled while reporting stories and it is an effective tactic. You are contained in an area with no food and no toilet facilities for hours at a time. It is also very annoying for those who want to make a statement of support and then leave. You can't. It is even worse for passersby who just happen to get caught in the middle, as frequently happens. They get stuck inside as well
On May Day 2001 a group of protesters were kept inside a kettle at Oxford Circus in the heart of London for many hours. A group of them - including several who were not part of the protest but just happened to be in the wrong place at the wrong time - sued the Metropolitan police, and lost when the case finally came up for judgment in the High Court in London in 2005. They then took their suit the European Court of Human Rights in Strasbourg.
Today the Human Rights justices agreed with the British court by a 14 to 3 vote, Their statement said, " … 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."
It is interesting to note that the ECHR is a favorite whipping boy of British Conservative eurosceptics, who regard it in much the same way as American conservatives regarded the Warren Supreme Court. They would have expected the court to back the protesters against the police. So far I haven't seen any comment from right wing papers praising the judges wisdom for backing the forces of law and order.
In one way the judgment is irrelevant. Protest has moved into new forms. The Occupy movement in a sense self-kettled. Setting up encampments gave the authorities many more head-aches than simply policing a march.