Thousands of people found by the high court to have been illegally detained for hours by police at a central London protest may sue Scotland Yard for false imprisonment.

The high court has ruled that the Metropolitan police had broken the law in the way it kettled up to 5,000 demonstrators at the G20 protests in April 2009.

The judges heard police used the tactic of mass detention against protesters that they accepted were peaceful, with officers meting out punches to the face, slaps and shield strikes as they tried to move a demonstration against climate change.

Judges found that the force used by police was "unjustified", criticised "imprecise" instructions given by senior officers about releasing innocent people, and said the mass detentions for five hours were an unlawful deprivation of liberty under article 5 of the European convention on human rights.

The case was brought by Josh Moos and Hannah McClure, who were among the crowd held by police.

The judgment does not strike down the police tactic of kettling, or mass detention, but it will be seen as a rebuff to the Met and places police chiefs on notice that the courts will intervene in favour of the rights of protesters over claims that draconian tactics are necessary to prevent violence. The Met announced it would appeal against the judgment.

The solicitor who brought the case, John Halford, said the ruling opened the police up to thousands of lawsuits seeking damages for the unlawful behaviour. He said: "Everyone in the kettle has a claim for false imprisonment and damages as they were detained against their will."

The case concerned the G20 protests in London on 1 April 2009, during which Ian Tomlinson, a bystander, died after being struck by an officer. There were several demonstrations in the area that day, but the court case deals with a Climate Camp protest in Bishopsgate. Police said they put in a containment of the camp, fearing people from a sometimes violent protest outside the Royal Exchange may join it.

In their judgment Sir Anthony May, president of the Queens Bench Division, and Mr Justice Sweeney said police had failed to meet the conditions for the mass detention of protesters to be lawful: "To be justified in law as being the lawful exercise of the common law power to take reasonable steps to prevent a breach of the peace and as not constituting an unlawful deprivation of liberty under article 5 of the convention, the police had reasonably to apprehend an imminent breach of the peace …"

But this was not the case, the judges found, citing the fact the two protests were relatively far apart. Those who left the Royal Exchange took two hours to reach the edge of the Climate Camp protest. The judges said the risk of violence was not imminent, and that was the threshold of danger to public order needed to justify the tactic of kettling. "There was such a risk, but it was at that stage only a risk; and it was not, in our judgment, a risk of imminent breaches of the peace sufficient to justify full containment at the Climate Camp."

For the Met, policing protests has seen them criticised on several fronts, as too heavy handed at G20,and then underprepared at last year's student protests.

The high court judgment found use of force at G20 was excessive, and the rights of innocent protesters were infringed: "It is evident that there were instances of unduly inflexible release [from the containment] and instances of unnecessary and, we think, unjustified force in the pushing operation."

The judges also criticised instructions a police chief said he gave to officers to release the innocent from the detention: "They were very general and imprecise and may not have been fully conveyed to individual officers, some of whom appear not to have been trained for crowd control operations of this kind."

The judges found a police decision to disperse the crowd to avoid it remaining overnight was lawful.

The Met said: "We believe that this issue is so important for the police's ability to prevent disorder within protest that we will be appealing. The MPS believes that the Bishopsgate containment prevented further scenes of violence and criminal damage occurring on 1 April 2009."

Dee Doocey, a Liberal Democrat member of the Metropolitan Police Authority, said: "We must have authoritative and crystal clear guidance on who can trigger the use of kettling … the Met must urgently address the insufficient training that police officers receive in the use of shields."

Jenny Jones, a Green party member of the Metropolitan Police Authority, said: "They have used kettling illegally, and sometimes violently, on peaceful demonstrations since May 2001. Kettling takes away the human rights and civil liberties of innocent, legal protesters."