Scotland Yard is facing legal action over claims that officers "falsely imprisoned" and assaulted schoolchildren during a tuition fees protest in London last month.

In what is believed to be the first lawsuit taken against police in connection with the violence, lawyers from human rights group Liberty have notified the Metropolitan Police of legal action involving minors who suffered "inhuman and degrading treatment" during a protest on 24 November.

The organisation claims the treatment of children amounted to a breach of their human rights after they were "kettled" by officers during the demonstrations for up to nine hours in cold conditions, without food, and were denied medical help despite some of them suffering injuries, including at least two fractures.

The claim is on behalf of three young protesters, one of whom is a 15-year-old whose foot was broken after allegedly being struck by an officer when trying to leave a police kettle and who claims she was subsequently refused medical help. Another is a 17-year-old London student who became so distressed inside the "kettle" that her father said she came away suffering from shock. The third is Rory Evans, 19, whose ankle was broken during a crowd surge among protesters contained between police lines.

Lawyers believe the Met breached the European convention on human rights on at least four counts. The case is believed to be the first of what many observers believe could be a number against police over the protests.

The 15-year-old claimant, a GCSE pupil who was wearing her school uniform, describes how she became anxious while "kettled" and decided to go home. The teenager was climbing a gate to leave when an officer pulled her down and struck her.

A letter to Scotland Yard's legal team states: "The police officer continued to pull her down, causing her to fall on to the floor. She picked herself back up and the police officer then hit her hard on her foot with a baton. She was then alone in the 'kettled' area and barely able to walk unassisted." "She was extremely cold and frightened and in a great deal of pain," the letter adds.

The 17-year-old, an A-level student, joined the protest and was kettled within 15 minutes of arriving in Whitehall. For six hours she unsuccessfully asked officers to allow her to leave because she was desperate to go to the toilet. At 6pm, portable toilets were delivered outside the "kettle", but after the teenager was allowed to use them she was escorted back inside the crowd. She has described seeing a woman pleading to be released because she felt nauseous. Later she was escorted from the kettle, vomited by the side of the road and was taken back into the kettle without receiving any medical attention.

After seven hours police said she could leave when her father turned up.

The girl, who cannot be named for legal reasons, told the Observer: "It's disappointing that young people had their opportunity to express themselves taken away. There are not many positive things for young people who are categorised as yobs and will be forced to pay ridiculous amounts for university. The police tactics made a mockery of pluralism in democracy."

The final case involves Evans, a recent school leaver who described how people "kettled" in Whitehall resembled a "large tide" against lines of police with officers pushing back. He said people started to fall and he became trapped, with other demonstrators falling on his ankle and causing it to break. Evans noticed young people in school uniform who had also fallen. In serious pain, the teenager was eventually released from the kettle but, although he asked police, they did not seek medical attention for him nor know where to find assistance.

Emma Norton, legal officer at Liberty, said: "Policing demonstrations is no easy task but the police must distinguish between the law-abiding majority and the handful intent on violence. Our three young clients came away from November's march distressed, and, in two cases, with broken bones.

"The tactic of 'kettling' large groups so that peaceful protesters and passers-by are trapped for hours alongside more troublesome elements exacerbates tensions and creates a risk to public safety."

Scotland Yard has justified "kettling", saying it was crucial to contain people and the threat of disorder while minimising the use of force. Last week the Metropolitan Police commissioner, Sir Paul Stephenson, said officers had to deal with "unrestrained violence" at the protests. Discussing his officers' actions, he said "things happen in violent disorders" and he regretted any injuries caused. He said any complaints about police conduct would be investigated.