Two grandmothers from Yorkshire face up to a year in prison after becoming the first people to be arrested under the Government's latest anti-terror legislation.

Helen John, 68, and Sylvia Boyes, 62, both veterans of the Greenham Common protests 25 years ago, were arrested on Saturday after deliberately setting out to highlight a change in the law which civil liberties groups say will criminalise free speech and further undermine the right to peaceful demonstration.

Under the little-noticed legislation, which came into effect last week, protesters who breach any one of 10 military bases across Britain will be treated as potential terrorists and face up to a year in jail or 5,000 fine. The protests are curtailed under the Home Secretary's Serious Organised Crime and Police Act.

Campaigners expressed their outrage yesterday at Charles Clarke's new law, which they say is yet another draconian attempt to crack down on legitimate protest under the guise of the war on terror. In October last year a protester in Whitehall was convicted for merely reading out the names of British soldiers killed in Iraq. And at the Labour Party conference in September the Government suffered severe embarrassment when Walter Wolfgang, a veteran peace activist who survived the Nazis, was detained for heckling Jack Straw.

Mrs John and Mrs Boyes, who have 10 grandchildren between them, were held by Ministry of Defence police after walking 15ft across the sentry line at the United States military base at Menwith Hill in North Yorkshire. They were held for 12 hours before being released on police bail. They will learn whether they are to face prosecution when they return to Harrogate police station on 15 April.

"We thought this was a really important issue and we just had to challenge it," said Mrs John, who was nominated for a Nobel Peace Prize last year. Mrs Boyes, who was cleared by a jury at Manchester Crown Court in 1999 of causing criminal damage to a British nuclear submarine, said: " I am quite willing to break the law and prepared to be charged and to go to prison. The Government thinks it can do whatever it wants and that it has a passive public which accepts whatever it throws at it. I find it very worrying."

The women, who have been arrested more than a dozen times between them, went equipped with a hammer and a small pair of bolt cutters as well as placards declaring their opposition to the new law. They had prepared statements denouncing United States military policy and expressing their support for the people of Diego Garcia and the Chagos Islands, who were evicted from their homes to make way for US military bases.

As well as Menwith Hill, the sites covered under the new law include Fylingdales, the early warning station on the North York Moors and the US air bases at Mildenhall and Lakenheath in East Anglia. From next week the powers will also cover three nuclear sites - Aldermaston in Berkshire, its research facility at neighbouring Burghfield and the Devonport naval base at Plymouth. The Government's decision suggests it is already preparing for the protests that would follow the expected decision to replace Trident with a new generation of nuclear weaponry.

Similar restrictions will be announced soon on selected non-military sites such as royal palaces and government buildings. The Ministry of Defence said the sites had been chosen because they had been the scene of regular protests. A spokeswoman said: "Persistent activity by protesters places them at risk of being mistaken for terrorists. It also unnecessarily diverts police resources ... People will still be allowed to protest outside sites. This legislation is about keeping police focused on the job they are paid to do."

Kate Hudson, who chairs the Campaign for Nuclear Disarmament, said: " The Government has a responsibility to safeguard its citizens - we would be the first to argue that. But there is a very fine line between protecting people and introducing legislation that is an infringement of civil liberties. In recent legislation the Government has got on the wrong side of that fine line."

Shami Chakrabarti, the director of Liberty, said: "When does a peaceful protester become a trespasser? In a free society, when does he become a criminal? In Britain in 2006, only one man - the Home Secretary - will now decide instead of Parliament and the court. Just when our politicians lament the demise of participatory democracy they increasingly criminalise both free speech and protest."

Mrs John described the new law as a "kick in the teeth for the Magna Carta" and said the need for opponents of the Government to take direct action was greater now than ever. "We have seen two million people standing in Hyde Park and Tony Blair had no compunction in ignoring them. Even though there are huge numbers of people who oppose what the Government is doing, the only effective protests have been where direct action is taken. We have to demonstrate at the bases where the killing capacity exists - we have to attack it at source. These are the eyes and ears of the US war fighting machine and they are on our soil."

Before Mr Clarke's announcement military police only had the power to escort protesters off the military sites and prosecute them for civil trespass.

Gagging orders

John Catt

AGE: 81

CRIME?: Wearing an anti-Blair T-shirt in Brighton during the Labour conference.

WHAT HAPPENED: He was stopped under section 44 of the 2000 Terrorism Act as he walked towards the seafront for an anti-war demonstration outside the conference. His T-shirt accused Mr Blair and George Bush of war crimes. He was released after signing a form confirming he had been questioned. The police record said the purpose of the stop and search was "terrorism" and the official grounds for intervention were "carrying plackard + T-shirt with anti-Blair info" (sic).

Walter Wolfgang

AGE: 82

CRIME?: Heckling Jack Straw, the Foreign Secretary, during his speech to the Labour Party conference.

WHAT HAPPENED: The veteran peace activist shouted "That's a lie" as Mr Straw justified keeping British troops in Iraq. He was manhandled by stewards out of his seat and ejected from the Brighton Centre. When he tried to re-enter he was briefly detained under Section 44 of the 2000 Terrorism Act. Amid the disastrous publicity, senior ministers, from Tony Blair down, apologised.

Maya Evans

AGE: 25

CRIME?: Protesting over British casualties in Iraq.

WHAT HAPPENED: Standing on the Cenotaph in Whitehall, she read out a list of soldiers killed in Iraq. She was arrested under the Serious Organised Crime and Police Act 2005, which requires police permission to make a protest within one kilometre of Parliament. She was given a conditional discharge after being found guilty. Lord Falconer of Thoroton, the Lord Chancellor, later denied that the prosecution was an "undue infringement" of individual liberties.

Flt Lt Malcolm Kendall-Smith

AGE: 37

CRIME?: Refusing to serve in Iraq.

WHAT HAPPENED: The RAF doctor served in Iraq twice, but refused to return for a third spell of duty last June. He argued that the military action was not justified as Iraq had not attacked the UK or one of its allies. He is being court-martialled, facing five charges of refusing to comply with an order. After a pre-trial hearing rejected his argument that the orders were unlawful, the court martial will open at Aldershot next week.

Brian Haw

AGE: 56

CRIME?: Maintaining an anti-war vigil outside Parliament.

WHAT HAPPENED: Mr Haw has become a permanent fixture in Parliament Square since June 2001, when he erected a series of placards berating Tony Blair and President George Bush. The Serious Organised Crime and Police Act 2005, was designed mainly with his vigil in mind. But the High Court ruled that the legislation did not cover his protest as it could not be applied retrospectively. The Government is appealing against that decision.