Anti-nuclear activists are among those taking part in a major demonstration in central London today. Kate Hudson, general secretary of the Campaign for Nuclear Disarmament, said mass mobilisation was key to persuading the new Government not to renew the Trident nuclear weapons.
Lindsey German, convenor of the Stop The War Coalition, meanwhile said: "I will be marching because I am fed up with all the main parties saying they are going to promote austerity. They have made the rich richer and the poor poorer and they're going to continue."
Russian submarines are likely to have gained "valuable intelligence" on the UK's nuclear deterrent since the scrapping of the Nimrod maritime patrol aircraft, key RAF figures have warned.
In a letter to the Daily Telegraph, five retired senior officers warned that Britain's lack of submarine hunting aircraft has given the chance for "intruders" to snoop in territorial waters and potentially track Trident missile carrying Vanguard-class nuclear submarines.
The officers said:
Without maritime patrol aircraft surveillance, opportunities for intelligence-gathering by such 'intruders' can only prejudice the security and effectiveness of our strategic deterrent.
Indeed, it would be surprising if valuable intelligence had not already been acquired by the Russian Navy since the Nimrod force was grounded in March 2010.
The safety of the UK's Trident nuclear weapons is to be discussed in Parliament today, after the SNP secured a debate slot.
The party wants to press UK government on recent claims made by a whistleblower concerning the state of the weapons programme.
Able Seaman William McNeilly, 25, caused a manhunt after he published an 18-page report online containing a series of allegations about nuclear submarines based at Faslane, which he called a "disaster waiting to happen".
Alex Salmond, the party's foreign affairs spokesman, said: "Trident is a key issue for people in Scotland. It is bad enough that Scotland is forced to house these weapons of mass destruction but these alleged breaches of security are deeply worrying - there must be absolutely no complacency."
It would be "irresponsible folly" for the next Government not to renew Trident, a group of former defence and security chiefs has warned.
Writing in The Times (£), the 20-strong group, including former GCHQ director Sir David Omand and former head of the Royal Navy Admiral Sir Jonathon Band, said that a decision against renewal would be "irrevocable".
In an uncertain world where some powers are now displaying a worrying faith in nuclear weapons as an instrument of policy and influence, it would be irresponsible folly to abandon Britain's own independent deterrent.
Submarines would have to stop patrolling the seas straight away, as credibility in the system would be lost, the group added.
The cost of replacing Trident, a system of submarine-based nuclear missiles, based on four boats is estimated at between £17.5 billion and £23.4 billion.
The SNP, Greens and Plaid Cymru are in favour of scrapping Trident, in contrast to the Labour and Conservative position.
A large number of activists have arrived at Faslane navy base in Scotland to take part in a protet against the Trident nuclear deterrent.
STV News reports that hundreds of protesters began gathering outside gates at the base from 7am in an attempt to stop workers from entering, with the blockade due to last until 3pm.
The Bairns Not Bombs anti-nuclear demonstration from from Scrap Trident Coalition aims to see the closure of the Royal Navy base, is home to the UK's nuclear weapons system.
Dame Vivienne Westwood, Frankie Boyle, and Massive Attack are among leading figures in music, the arts and science calling for the UK to scrap its nuclear deterrent.
Nobel prize winner Professor Peter Higgs, former Royal Society president Sir Michael Atiyah and US linguist Noam Chomsky also put their names to a letter published in the Observer suggesting Britain should "move on from Trident".
Launched by political group Compass, the letter claims polling data suggested nuclear disarmament is a "majority popular demand" across the country.
Last week, Labour leader Ed Miliband reacted angrily after Conservative Defence Secretary Michael Fallon claimed that he would "stab the United Kingdom in the back" over the renewal of the Trident nuclear deterrent to secure a post-election deal with the Scottish National Party.
The way the Conservatives' tried to nail the day's election agenda to the issue of Britain's nuclear deterrent has ended with Labour accusations that they'd descended into the politics of the gutter. The row was about the way the Defence Secretary, Michael Fallon, criticised the Labour leader Ed Miliband.
He said Mr Miliband has stabbed his brother David in the back to get the leadership and was ready to do the same to the United Kingdom over its security. He was claiming there could be some sort of deal with the SNP to scrap our nuclear submarines.
On the Conservative campaign trail ITV News' James Mates assesses the fall-out:
Trident became a major campaign bone of contention today, because of the SNP's declared determination to scrap the nuclear deterrent. Nicola Sturgeon has told ITV News it's a "blood red line" that cannot be crossed, in any future Parliamentary deal with the SNP.
From Faslane, ITV News correspondent Martin Geissler reports:
The campaign got nasty today: defence was the theme, but offence was taken. The Tories said Labour would abandon the Trident nuclear deterrent, to cut a deal with the SNP. Ed Miliband would be "stabbing the nation in the back" the Defence Secretary said, just as he'd stabbed his own brother in the back. But Mr Miliband deployed calm fury saying Michael Fallon had "demeaned himself and his office." It was lies and a deceit, said Labour, dragging the campaign "into the gutter".
Ukip leader Nigel Farage and Green party leader Natalie Bennett have condemned a personal attack on Ed Miliband by the Conservatives.
Defence Secretary Michael Fallon made reference to the Labour leadership contest between Mr Miliband and his brother David, and suggested he would 'stab the UK in the back' over Trident.
Mr Farage said he fears the election campaign is becoming "an American, negative, shouting match".
Speaking in Kent, he said: "The attack on Ed Miliband was very, very personal, calling him a backstabber.
"I just fear that we have an election campaign that is turning into an American, negative, shouting match between two parties, and I don't think the public like it.
"I certainly don't."
During a visit to Norwich, Ms Bennett said: "A very important issue has been raised in the debate about Trident today but the way in which it was raised is deeply damaging.
"This kind of personalised attack is the kind of Punch and Judy politics that is really damaging our political fabric."