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."
The main political parties are wrangling over whether the UK should spend billions renewing its nuclear deterrent.Read the full story ›
David Cameron earlier defended Michael Fallon's personal attack on Ed Miliband, but has now said "it's important to be respectful".
Speaking at a PM Direct event at the National Grid training centre in Nottinghamshire, Mr Cameron was asked about the Defence Secretary's claim that the Labour leader "stabbed his brother in the back" over their 2010 leadership contest.
I think we should try to be respectful in the way we conduct our political debate.
I think, generally speaking, in Britain we are. It is a feisty democracy. We have big arguments, big disagreements.
He added that Mr Fallon had been making a key point about security and was right to "raise it in a pretty frank way, as he did".
Nick Clegg has told ITV West Country he "can't believe" the Conservatives regard the difference between three and four nuclear submarines is the most important issue facing nation.
ITV News caught up with the Lib Dem leader as he visited Bodmin Hospital this morning. The full interview will broadcast on ITV News West Country later.
Nick Clegg says the reason the Conservative and Labour parties are trading personal barbs is because they are "panicking."
In elections people say disobliging things.
...The reason you're seeing more nasty language is because both parties are panicking they're not going to win a majority.
What the country now faces is the prospect of David Cameron in Downing Street with Nigel Farage and at the mercy of the swivel-eyed brigade on the right of his party, or Ed Miliband at the mercy of Nicola Sturgeon and dancing to the tune of Alex Salmond.
That's why they are panicking and that's why they are resorting to ever more vituperative language against each other."