London police were authorized to fire plastic bullets to quell riots and dozens of letters were sent out to activists warning of arrest for "criminal or antisocial behavior" ahead of planned mass student protests on Wednesday.
Some 4,000 officers will be on the streets during the march in central London, which authorities fear may be hijacked by anarchists and troublemakers.
The extra measures come after student protests last year descended into chaos. Rampaging demonstrators smashed Conservative Party headquarters on Nov. 10, and on Dec. 9 Prince Charles' limousine was surrounded and rocked in a student protest.
London officers told the Daily Mail online that they do not intend to deploy plastic bullets, and that they would only be used in an extreme case in which other tactics failed and officers' lives were at risk. Plastic bullets have never been used on Britain's mainland, though in the past have been linked to deaths in Northern Ireland.
The police officer in charge of the operation, Commander Simon Pountain, told the Mail that armored vehicles would also be on standby in the event protests escalated to violence.
The letter from Metropolitan Police to activist students, some as young as 17, arrived in student mailboxes on Tuesday, the Guardian reported.
"It is in the public and your own interest that you do not involve yourself in any type of criminal or antisocial behaviour," the one-page letter stated. "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."
The mass protest was being organized by the National Campaign Against Fees and Cuts, which said it expected 10,000 people to join in a march from Bloomsbury in central London to the center of the city.
Some activists from the anti-capitalist Occupy London Stock Exchange movement in London said they would join the march, according to local reports.