Politicians including Jeremy Corbyn and Nicola Sturgeon have condemned plans to renew the UK's Trident nuclear weapons system, at a London rally.
Thousands of people joined a march and the rally in Trafalgar Square.
Labour leader Mr Corbyn said peace was not achieved by "planning for war", while Scottish First Minister Ms Sturgeon described Trident as "immoral" and "impractical".
The MoD estimates renewing the Trident system will cost £31bn over 20 years.
That involves acquiring four new submarines to carry the missiles, while a further £10bn has been earmarked for any unexpected costs.
Mr Corbyn told campaigners: "We live in a world where so many things are possible. Where peace is possible in so many places.
"You don't achieve peace by planning for war, grabbing resources and not respecting each other's human rights."
He added: "Today's demonstration is an expression of many people's opinions and views. I'm here because I believe in a nuclear-free Britain and a nuclear-free future."
Mr Corbyn has asked shadow defence secretary Emily Thornberry to carry out a review of Labour defence policy, including its stance on Trident renewal.
The Labour leader supports unilateral disarmament but some members of the shadow cabinet, including shadow foreign secretary Hilary Benn, are in favour of Trident.
SNP leader Ms Sturgeon said the norm in the world today was to be nuclear-free.
"It is the exception to the rule to possess nuclear weapons, let that message ring out loudly and clearly," she said.
"The use of Trident nuclear weapons would bring about human devastation and suffering on an unimaginable scale."
Plaid Cymru leader Leanne Wood said: "The world has been and continues to be an unstable and unpredictable place but there are some values that we should hold on to through peace, through war, through instability and unpredictability.
"It is never acceptable, it is never justifiable to unleash weapons of mass destruction on a population. Nuclear weapons belong in the dustbin of history alongside the Cold War."
Former Green Party leader Caroline Lucas described nuclear weapons as "a Cold War relic".
"To contemplate using nuclear weapons is both illegal and immoral," she said.
Actress and campaigner Vanessa Redgrave also spoke at the Stop Trident event, which was organised by the Campaign for Nuclear Disarmament.
Supporters say Trident is indispensable for protecting the UK's security, and the nuclear defence industry is also a major employer.
Replacing Trident was a Conservative manifesto pledge at the general election.
A Commons vote on Trident renewal is expected to be held later this year.