Political leaders have headed off on the campaign trail after Prime Minister Gordon Brown announced the UK general election would be held on 6 May.
He said he would seek a "clear" mandate to continue the "road to recovery", as Labour bids for a fourth term.
David Cameron, whose Conservative Party has been ahead in the polls, said they offered "hope" and a "fresh start".
Nick Clegg, leader of the UK's third biggest party the Liberal Democrats, said only they offered "real change".
Shortly after announcing the date at Downing Street, Mr Brown boarded a train and headed to Kent to meet voters at a supermarket in Rochester.
Mr Cameron headed to a hospital in Edgbaston, Birmingham, before addressing a rally in Leeds while Mr Clegg met young people in Watford.
All three are leading their parties into a general election for the first time.
Announcing the widely-predicted 6 May election date after meeting the Queen, Mr Brown 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."
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."
Mr Brown 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.
Addressing party supporters later in Leeds, he attacked Labour's economic record and said the public should "never forget" that Gordon Brown presided over the longest recession in more than 60 years.
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".
He said it would be a choice "between more of the same from the old parties... or real change, something different from the Liberal Democrats".
"I think we just need to do something new this time," he added.
The election campaign will be the first to feature live television debates between the three main party leaders in the UK.
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.
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.
He said the SNP would work with Plaid Cymru in the event of a hung parliament to try to secure the best deals for Scotland and Wales.
Plaid Cymru's leader Ieuan Wyn Jones added: "We want to secure the best deal for Welsh communities in this election. And in a situation where no party has overall control in the next parliament then we will be fighting for a fairer funding system for the people of Wales."
DUP leader Peter Robinson told the BBC the election would be a "defining moment" for Northern Ireland: "It's an opportunity for people to decide whether they want to move forward, whether they want to continue the progress, I think that will be the main issue, though the hung parliament issue will loom large."
But he said, in the event of a hung parliament, the DUP would not be "in hock" to any of the main parties.
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, a post-war record number of MPs are standing down at the election - 144 - so there will be a lot of new MPs in the next Parliament.
Parliament will not be officially dissolved until Monday 12 April. MPs will have until close of business on Thursday to get remaining legislation, that the parties can agree on, through Parliament - a process known as the "wash-up".
Commons leader Harriet Harman said this would include all stages of the Finance Bill, which enacts the Budget, and further debate on the Digital Economy Bill, constitutional reform and crime and security legislation.
The government will drop plans for a referendum on changing the voting system and to phase out remaining hereditary peers.
MPs will not return until Tuesday 18 May - later than the traditional start date of the week after the election. A modernisation committee recommended a 12-day gap after the election 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.