Met writes to people previously arrested for public order offences even if they were cleared or charges were dropped

Police have sent dozens of letters to anti-cuts activists as young as 17 warning them of the consequences of attending a student demonstration planned in central London on Wednesday.

The letters, which activists say have had a severe chilling effect on protest, have been sent to anyone arrested with previous public disorder offences even if they were later cleared or charges were dropped.

At a briefing Met officials said that letters would only be sent to those who had been convicted of offences but a spokesperson from the Met later admitted this was incorrect and confirmed that anyone who had been arrested in the last year in relation to an "austerity related" protest had been sent the warning.

The single page letter, which arrived through letter boxes on Tuesday, reads: "It is in the public and your own interest that you do not involve yourself in any type of criminal or antisocial behaviour. We have a responsibility to deliver a safe protest which protects residents, tourists, commuters, protesters and the wider community. Should you do so we will at the earliest opportunity arrest and place you before the court."

Signed by Simon Pountain, the Met commander leading Wednesday's operations, the letter goes on to warn of detrimental effects of conviction on their chances of employment and says that if people find themselves near disorder they should move away at the earliest opportunity.

Essex sixth form student Tyler Perkin, who previously had all charges dropped in relation to a peaceful UKuncut protest inside Fortnum and Mason store in March this year said that he was horrified by the fact that he was still on a police database.

The warning letters come on the back of a police briefing in which police highlighted that they had authority to fire rubber and plastic bullets – collectively known as baton rounds – at protesters.

It is believed to be the first time baton rounds have been pre-authorised for a planned protest march on the British mainland.

Perkins believed that along with briefings about rubber bullets, the police were trying to scare people away from the protest.

"It's a disgusting attempt to try and scare us off from protesting," he said.

"I know another youth defendant has received [the letter] and I have seen another five people who have tweeted [that they have got] it," Perkins said.

"It's also disgusting that we are all been kept on some database even though we haven't been charged … even though my case has been dropped they're still using it against us," Perkins added. He said that he had also been visited by police in conjunction with this summer's riots even though he had no connection with them.

Lawyer Rhona Friedman told the Guardian that two of her clients had received police letters and that she also feared police databases were being employed to stop innocent young demonstrators from taking to the street.

"The letter is being sent to young people who have not been convicted of any criminal offence but whose names must appear on the police intelligence data base of those engaged in protest.

"Whether deliberate or not the message is being received as being less about public protection and more about sending a chilling Big Brotheresque warning about the consequences of participation in public demonstrations."

Organisers of the march, which will set off after midday on Wednesday and join Occupy London Stock Exchange protesters who are camped around St Paul's cathedral, have written to police warning them that children will be part of the demonstration after they learned of the authorisation of rubber bullets.

In Northern Ireland baton rounds have killed 16 people in past decades but they have never been used in public order situations elsewhere in Britain. Eight of those deaths involved children.

Baton rounds are regularly used in the UK. Since 2002 they have been used by police to stop fleeing vehicles or in cases where Taser guns could not be deployed. Their most recent use was by Norfolk police on 9 August during a domestic incident, according the Association of Chief Police Officers (Acpo).

During the height of this summer's riots the Acpo chief, Sir Hugh Orde, spoke out against rubber bullets saying their use would not be sensible and their deployment had to be kept in proportion.

Talking down their use on the eve of the march, a police spokesperson said that the Met had "no intention of deploying" baton rounds during the demo.

The Met also pointed out it had sent letters out to convicted criminals in the past including for the Notting Hill Carnival but it was unclear whether this had been done before a protest and to those who had only been charged with an offence.

The National Campaign Against Fees and Cuts (NCAFC) says it expects around 10,000 people to attend the march, which comes on the first anniversary of the demonstration that saw the Tory party headquarters on Millbank trashed by student activists.

The Guardian has learned that groups engaging in direct action are likely to break off from the march at different points, causing disruption which may become a focus of police attention.

March organiser and NCAFC co-founder Michael Chessum said: "What the police are engaged in is a cynical attempt to stop people from attending the demonstration and to pre-criminalise the protest. That's not new as such but it is new that it is as extreme as this.

"They are trying to ramp up the atmosphere around the protest before it begins … they should be getting on with their job to facilitate protest," he said.

Teachers' associations have warned pupils not to miss lessons to attend the march.

Russell Hobby, general secretary of the National Association of Head Teachers, said pupils would be disciplined for taking unauthorised absence to go to the march. "With talk of the police using baton rounds, it just doesn't seem a good idea for anyone, particularly the young and vulnerable, to be involved."

Brian Lightman, general secretary of the Association of School and College Leaders, said it was not acceptable for pupils to "absent themselves without permission".

"Teachers would not expect pupils to miss lessons to go to this march. We would encourage pupils to discuss with their teachers how they can exercise their democratic rights."

The general secretary of the University and College Union, Sally Hunt, said she empathised with the demonstrators aims saying they were "understandably angry at attempts to privatise our internationally-respected higher education system to people only interested in the possibility of a fast buck".

• Extra research by Ami Sedghi