More than 100 protesters have gathered at an atomic research base claiming any plans to update the UK's nuclear defences are unlawful.
Many of the protesters turned up dressed as UN weapons inspectors
Greenpeace campaigners gathered at Berkshire's Aldermaston atomic weapons establishment (AWE), to voice anger at renewal of the Trident system.
MPs are to vote on whether Trident will be replaced next year but protesters claim replacement work is under way.
Greenpeace claims this breaches the nuclear non-proliferation treaty.
The AWE demonstration was given the name the "world's biggest ever weapons inspection".
Many of the Greenpeace protesters turned up dressed as UN weapons inspectors.
Among them was Anita Roddick, founder of Body Shop, who said: "Sixty years ago we invented a way to extinguish life on Earth at the touch of a button, which was one of the less impressive things human beings ever did.
"Now Tony Blair has the chance to leave an historic legacy to the world by making Britain the first UN Security Council member to say we no longer want or need these monstrous weapons.
"If he doesn't, he'll break international treaties and send an invitation to every nation on Earth to join the nuclear club."
AWE is the headquarters of Britain's nuclear development programme
The claims are based on advice given to the campaign group by Phillippe Sands QC, a critic of government plans to upgrade the Trident system.
By developing new nuclear weaponry, Greenpeace claims, Article 6 - an agreement to nuclear disarmament - of the Nuclear Non-proliferation Treaty is being broken.
AWE, which is the headquarters of Britain's nuclear development programme, said it did not want to comment on the future of Trident, but that it recognises people's right to demonstrate peacefully and lawfully.
Police officers were sent to monitor the research facility protest and one arrest was made.
Last month, a Ministry of Defence spokesman said Trident missiles were not being replaced at the site, but equipment needed to be replaced along with retiring staff.
Vote in 2007
Building work and the creation of hundreds of jobs led Greenpeace to claim new nuclear developments were under way at the site.
Earlier this month Downing Street revealed MPS would vote early next year on whether Trident, whose working life is due to end in 2024, should be replaced.
Ministers are to outline their favoured option - expected to be a replacement for Trident - in a white paper to be published in December.
Prime Minister Tony Blair and Chancellor Gordon Brown have both indicated their support for retaining an independent nuclear weapons system.