A leading legal aid firm has accused the Metropolitan police of criminalising a generation of students for taking part in the protests against tuition fees.

Hodge Jones & Allen, which specialises in protest litigation, argues that the Met has handed out an excessive number of cautions for the offence of aggravated trespass to students.

The offence is committed when someone trespasses on open land and causes disruption or intimidates. It results in a criminal record.

The Met said it had issued 26 cautions for aggravated trespass after the 10 November protest against higher tuition fees. It said that no one else had been issued with a caution for this offence after subsequent protests, on 24 and 30 November and 9 December. However, up to 150 people were arrested in total at the four protests and lawyers say most of these are still awaiting a police decision and could still be issued with a caution for aggravated trespass.

Lawyers at the London firm said they were acting for a "significant number" of students arrested after the November and December protests. They argue that the offence of aggravated trespass is easily committed and is "being used by the Met to criminalise people for exercising their right to protest". They warn that students with cautions for the offence could be turned down by future employers.

"Conduct falling within the category of aggravated trespass can literally be anything that is potentially disruptive, such as playing a musical instrument," said Ruth Hamann, one of the firm's criminal lawyers.

Cautions are formally recorded on the police's national computer database as an admission of guilt to a relatively minor criminal offence.

It would be unlikely for someone with a caution on their record to be given another if they re-offended, Hamann said. Instead, they would be treated more harshly.

"This may dissuade some young people from attending subsequent protests for fear that they might be charged with an offence and required to attend court," she said.

She warned that students had to disclose any cautions when applying for jobs in teaching, law and medicine, among other professions. "While aggravated trespass might not be the sort of offence that would automatically make a person ineligible for a job, it may encourage an employer to favour another candidate over the candidate with a caution. Our concern is that, by using these wide discretionary powers, the Met are criminalising a generation of political activists."

Hamann said she thought further arrests would be likely after a major protest, being organised by the TUC, against cuts to education and public services in central London on 26 March. Tens of thousands of protesters are expected to take part.

Last week Scotland Yard announced that it had employed a "kettling manager" before the 26 March protest. The Met has been working on plans to avoid the violence that marred student protests last year and is poised to request assistance from neighbouring forces.

The TUC is working with Scotland Yard, and both insist they expect the protest to be peaceful.

A spokesman for the Met said decisions to caution people had not been made lightly. "To be cautioned, you have to admit the offence. We would not normally release someone who had admitted an offence without any action. We would, of course, follow any national guidelines on how to deal with young offenders."

Rees Johnson, a 20-year-old student from Newcastle who was issued with a caution after the 10 November a protest in central London last November said he would not be going to any further demonstrations out of fear that he might be arrested again. "It seemed as if the police could arrest people for just being there," he said. "If I found myself in trouble again, I wouldn't be entitled to a caution, so I won't be going to any more protests."