A decision is expected to be taken later this year on replacing the ageing submarines which carry the Trident missiles at an estimated cost of 31 billion pounds. Prime Minister David Cameron's Conservative government is in favour of it.
Thousands of people joined a protest in central London today against the renewal of Britain's nuclear weapons system Trident.
Scotland's First Minister Nicola Sturgeon and Jeremy Corbyn, the leader of Britain's main opposition Labour party, were set to address a rally following the march.
A decision is expected to be taken later this year on replacing the ageing submarines which carry the Trident missiles at an estimated cost of 31 billion pounds (USD 43 billion).
Prime Minister David Cameron's Conservative government is in favour, saying the system is vital to safeguard Britain's security.
"Disarming now would be a reckless gamble with our national security that would play into the hands of our enemies," Defence minister Philip Dunne said ahead of the march.
Labour is deeply divided on the issue between leftwingers like Corbyn, who want to scrap it, and some centrists who want to keep it.
Britain currently has four submarines in its Trident fleet, at least one of which is on patrol somewhere in the oceans 24 hours a day.
The government wants to replace those with four so-called Successor submarines, the first of which would enter service in the early 2030s.
March organisers insist there is growing popular opposition to the Cold War-era weapons system, which is based at a sea loch on the west coast of Scotland.
Ahead of the event, the chair of the Campaign for Nuclear Disarmament Kate Hudson said: "The extent of popular opposition is becoming clear.
"This will not be a gathering of the anti-nuclear clan - it will show the breadth of opposition." Scotland's ruling Scottish National Party (SNP), led by Sturgeon, wants to scrap nuclear weapons, meaning that any future vote in favour of Scottish independence would complicate the picture still further.