(BBC) - Prime Minister Gordon Brown has confirmed that the UK general election will be held on 6 May.
Flanked by his entire cabinet, Mr Brown said it was the "least well-kept secret of recent years".
He said Britain was on the "road to recovery" and urged voters not to put it "at risk".
But David Cameron said the Conservative Party offered a "fresh start", while Lib Dem Nick Clegg said only his party offered "real change".
Mr Brown made the announcement outside Downing Street, after a 20-minute meeting with the Queen at Buckingham Palace to seek the dissolution of Parliament.
'Ordinary background'
The prime minister - who faces his first election as Labour leader - said he wanted a "clear and straightforward mandate" to continue the work of economic recovery.
He said he would be travelling the country telling voters: "Britain is on the road to recovery and nothing we do should put that recovery at risk."
And he added: "We will not allow 13 years of investment and reform in our public services, to build up the future of these great services, to be put at risk."
He also said he would produce a plan to make politics more transparent and accountable.
Stressing his "ordinary middle-class background" he said Labour would "fight for fairness at all times".
Mr Brown said: "We will say to the British people: 'Our cause is your cause'," before adding: "Let's go to it."
But Mr Cameron said he offered a "modern Conservative alternative" and his party offered "hope, optimism and change" and a "fresh start".
"It's the most important general election for a generation. It comes down to this. You don't have to put up with another five years of Gordon Brown."
He criticised 13 years of Labour's "big government" and said it was time for the Tories' "big society" instead. He pledged to work for the "great ignored", who he described as "honest hard-working people" who "do the right thing".
"Let's get off this road to ruin and instead get on the path to prosperity and progress," he said.
Lib Dem leader Mr Clegg said the election campaign would not be a "two-horse race" between the two biggest parties, and people were "crying out for something different".
"All bets are off," he said.
"This is a choice now between the old politics of the two old parties and something new, something different, which the Liberal Democrats offer."
It will be the first time that all three have led their respective parties into a general election - none was a party leader at the last one in 2005.
Hung Parliament
It will also be the first campaign to feature live television debates between the three main party leaders.
BBC, Sky and ITV announced the first 90-minute debates would be on ITV on Thursday 15 April, the next on Sky on 22 April and the last on the BBC on 29 April.
After the announcement, Mr Brown headed to St Pancras station to take a train to Kent to begin campaigning. He started by meeting voters at a supermarket.
Mr Cameron travelled to Birmingham, where his first visit is to a local hospital, while Mr Clegg has started his campaign in Watford.
The three main parties - along with a host of other smaller parties - will be fighting for 650 seats, four more than currently exist because of constituency boundary changes.
SNP leader Alex Salmond, the first minister of Scotland, hopes his party will win 20 seats at Westminster. He told the BBC the other parties had "blown the gaffe" by outlining plans for deep cuts.
"In these circumstances the need for Scotland to have national champions in the SNP is greater than ever before," he said.
'Old politics'
Meanwhile Plaid Cymru's director of elections, Helen Mary Jones, said it was time for the "worn-out government" to go: "We're asking the people of Wales to think about it this time.
"Do they really want more of the same old politics of boys yelling at each other, or do they really want a representative that can really listen to them?"
Both parties say they could end up holding the balance of power, if there is a hung parliament.
To secure an overall majority, a party must win at least 326 seats. If no party succeeds in doing so, the result will be a hung Parliament.
After 13 years in power, Labour enters the election with a notional majority of 48 seats, meaning that a loss of 24 seats would see them lose their overall majority.
Whatever the result, the make-up of the House of Commons will change significantly following the election, with 144 MPs so far having announced that they will stand down.
Parliament will not be officially dissolved until Monday 12 April - MPs will spend this week getting remaining legislation, that the parties can agree on, through Parliament - a process known as the "wash-up".
MPs will not return until Tuesday 18 May - later than the traditional start date of the week after the election - after a modernisation committee recommended there should be 12 days between the events, to allow for a proper induction for new MPs.
Opinion polls timed to coincide with the announcement all suggest a Conservative lead over Labour, by differing margins.
An ICM survey for the Guardian indicates the Tory lead has dropped to just four points, with the Conservatives on 37%, Labour on 33% and the Lib Dems on 21%.
However a YouGov poll in the Sun and another by Opinium for the Daily Express suggest the Tories have opened up a 10% lead - the margin David Cameron is likely to need in order to win an outright majority on 6 May. The Sun has the Tories on 41%, Labour on 31% and the Lib Dems on 18%. The Express reports a 39/29/17 split.
