Thousands of people opposed to the renewal of the multibillion-pound Trident nuclear system marched through London in the biggest demonstration of its kind for a generation.
Anti-nuclear protesters from across the UK armed themselves with placards and banners as they braced the bitter winds whipping around the centre of the capital to spread their message.
The group - estimated by stewards to run into "many tens of thousands" - were joined on the marches by SNP leader Nicola Sturgeon and her Plaid Cymru counterpart Leanne Wood, with organisers hoping the event would send a strong message of growing support against renewing the nuclear weapons system.
The Ministry of Defence has estimated acquiring four new submarines to carry the Trident deterrent will cost £31 billion over the course of the 20-year procurement programme, with a further £10 billion set aside to meet any additional unexpected cost increases.
Addressing the crowd as the rally continued to gather in Trafalgar Square, Ms Sturgeon described Trident as "immoral" and "impractical".
She said: "It is the norm in the world today to be nuclear-free. It is the exception to the rule to possess nuclear weapons, let that ring out loudly and clearly.
"The use of nuclear weapons would bring about human devastation and suffering on an unimaginable scale."
She said the SNP would be making Trident a political issue at forthcoming elections.
Former Green Party leader Caroline Lucas described nuclear weaponry as "a cold war relic".
She said: "To contemplate using nuclear weapons is both illegal and immoral."
Labour leader Jeremy Corbyn was the last of more than 20 speakers - including senior union members and actress Vanessa Redgrave - to address the crowd.
Arriving fresh from a Labour hustings in the north of England, Mr Corbyn said: "If a nuclear war took place there would be mass destruction on both sides of the conflict.
"Everyone should think about the humanitarian effects on people across this globe if they're ever used.
"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.
"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.
"Thank you for coming to this demonstration, thank you for showing that you care and thank you showing you want a peaceful future for this country and the rest of the world."
The Labour leader favours unilateral disarmament but faces a showdown with some of his shadow cabinet, including shadow foreign secretary Hilary Benn, who said he believed in multilateral action.
The shadow cabinet has yet to decide how it will handle any Commons vote on Trident, expected later this year, ahead of the recommendations of a review of the party's existing support of renewal led by unilateralist shadow defence secretary Emily Thornberry.
Michael Dugher, who was sacked as shadow culture secretary in January's shadow cabinet reshuffle, led criticism of the leader's decision to attend.
"The Tories must think it's Christmas," he posted on Twitter alongside a CND post showing Mr Corbyn arriving at the rally.