Thousands of people have gathered in Berkshire to mark the 50th anniversary of a march to the Atomic Weapons Establishment (AWE) at Aldermaston.
Campaigners are concerned about the expansion of the site
The first march was held at Easter in 1958, shortly after the formation of Campaign for Nuclear Disarmament (CND).
Up to 5,000 people from across the UK have been arriving in coaches and by foot since the morning.
MoD police have warned that anyone who tries to breach the site perimeter will be arrested.
High profile faces, including fashion designer Vivienne Westwood and CND's 84-year-old vice president Walter Wolfgang, were at the Bomb Stops Here protest.
Mr Wolfgang was famously ejected from the 2005 Labour Party conference for heckling Jack Straw during a speech about the Iraq War.
In 1958, some 10,000 people marched from London to Aldermaston in protest at Britain's first hydrogen bomb tests 50 years ago.
The site is the headquarters of Britain's nuclear weapons programme.
Christopher Holden, who was 15 years old when he went on the first march, was at the event.
He said: "I just remember the great feeling of how worried everybody was about the nuclear issue during the Cold War.
"The issue hasn't gone away. It is still as important as it was then."
Christopher Holden went on the original march as a 15-year-old
AWE provides the warheads for Trident - the submarine-launched missile system that constitutes the UK's nuclear deterrent.
Organisers of Monday's march said the protest will highlight how the site is being expanded to develop the next generation of nuclear warheads.
The CND spokesman said coaches from more than 50 locations will bring activists from as far as Aberdeen and Penzance.
He said the protesters were planning to surround the base, which produces the warheads for the Trident nuclear weapons system.
The demonstration comes in the wake of recently-imposed Ministry of Defence by-laws restricting the right to protest on land surrounding AWE Aldermaston.
Aldermaston protests are an annual affair, including this one in 1960
CND, however, said police had agreed to lift the bylaw for the event. The spokesman said: "The main message is the anti-nuclear one but it is also very important that we actually have the right to protest.
"Although the civil liberties issue is secondary, it is very important. Police have agreed to lift the bylaws to allow the day-long event to go ahead."
CND said a delegation of survivors from the 1945 Hiroshima and Nagasaki bombings were marching to Aldermaston from the former US Cruise Missile base at Greenham Common, 10 miles away.
More than 150 officers from the Thames Valley, MoD and Hampshire police forces, including mounted officers were at Aldermaston to "facilitate a lawful and peaceful protest".
Supt Chris Shead, of Thames Valley Police, said: "CND have worked with us in negotiating the details of their planned demonstration and we are hopeful that this event will pass off peacefully, with no criminal activity and minimal disruption to the local community and AWE."