The general election will be held on 5 May, Tony Blair has formally announced.
Tony Blair said he would give a "positive vision" for Britain
Speaking after asking the Queen to dissolve Parliament next week, Mr Blair said Labour had a "driving mission" for a third term in office.
The Conservative and Liberal Democrat leaders pre-empted the announcement by starting nationwide tours of key seats.
Michael Howard accused Mr Blair's government of "losing the plot", while Charles Kennedy said he would focus on people's hopes, not their fears.
Mr Blair told reporters in Downing Street the election presented a "big choice".
"The British people are the boss and they are the ones that will make it," he said.
The Labour leader said he wanted to "entrench" economic stability and public services' investments, as well as ensuring people from all backgrounds could achieve their potential.
He then headed off by helicopter to make a speech in Weymouth, Dorset - part of Labour's most marginal seat.
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Earlier, Labour's candidate in Ribble Valley, Stephen Wilkinson, said he was defecting to the Lib Dems.
Four opinion polls published on Tuesday suggest Labour's lead over the Tories has slipped to between 2% and 5%. They suggest the Lib Dems trail the Tories by between 10 and 16 points.
But one of the polls also suggests the Tories are 5% ahead of Labour among those "certain to vote".
With the campaign under way, ministers must rush to get outstanding legislation through Parliament before it winds up, probably on Friday. It will be formally dissolved on Monday.
Backroom horse trading is happening between the parties over which bills of legislation can still be passed.
Commons Leader Peter Hain said he hoped 16 bills - more than half the number announced in last year's Queen's Speech - would have been passed before Parliament adjourned.
Mr Hain said plans for a new offence of incitement to religious hatred looked set to be lost.
The Tories say plans for identity cards are another "likely casualty".
But ministers reached a compromise to save plans to overhaul gambling laws by cutting the number of regional "super casinos" allowed from eight to one.
'Action or talk?'
As he launched his party's campaign in London, Conservative leader Michael Howard said voters faced a "clear choice".
"They can either reward Mr Blair for eight years of broken promises and vote for another five years of talk.
"Or they can vote Conservative to support a party that has taken a stand and is committed to action on the issues that matter."
Tuesday: Prime minister asks Queen to dissolve Parliament and announces a 5 May poll
Wednesday: Last prime minister's questions
Friday: Likely last day when Parliament sits
Saturday: Campaigning suspended for royal wedding
Monday: Parliament formally dissolved
Mr Howard later visited Sale and Birmingham, where there was a minor scuffle as Labour activists with anti-Conservative banners were manhandled away by Tory workers.
Lib Dem leader Charles Kennedy has visited Manchester, Newcastle, Leeds and Edinburgh on a whistle-stop tour of key seats to begin his campaign.
He told BBC News the election was "much more fluid" than ever before. He promised to shun his rivals' "negative campaigning".
"I'm not going to spend the next month just talking Britain down," he added.
Wales and Scotland
Plaid Cymru MP Elfyn Llywd said his party was the real opposition in Wales and there was no real difference between Labour and Tories.
Scottish National Party leader Alex Salmond said his campaign would focus on making Scotland matter to the election.
The Green Party said it was fielding 25% more candidates at this election on a "People, Planet, Peace" slogan.
The UK Independence Party said it was the only party who believed the UK should govern itself, independent of European Union controls.