To announce the poll date Mr Blair chose an inner-city "beacon" school in Bermondsey, south London, telling his audience: "Every vote in this election is precious. No one's support should ever be assumed. That is the strength of our democracy."
Tory leader William Hague insisted his party could win despite its poor showing in the polls and Liberal Democrats leader Charles Kennedy expressed relief that the long wait was over.
At lunchtime, Mr Blair had driven to Buckingham Palace to asked the Queen for a dissolution of parliament next Monday.
"We earned the trust of the people in 1997 after 18 long years of opposition. Today we have to earn that trust again.
'Humility and hope'
"I stand before you today with a sense both of humility and of hope."
" No government of change, in any country I can think of, has done it all in one programme, one budget, or one term "
The prime minister, addressing the audience beyond those in the school hall, said: "Four years ago you gave us, New Labour, a majority larger than I or anyone else believed you would.
"That large majority was never a reason to do the job quickly. It added the responsibility to do it properly, for the long term."
He appealed for a second term, saying: "No government of change, in any country I can think of, has done it all in one programme, one budget, or one term."
Tories 'ready to win'
He travelled to St Saviour's & St Olave's Church of England School with Education Secretary David Blunkett.
Mr Blair's announcement heralds the official start of his campaign to win a second term for Labour.
Opinion polls have consistently suggested that Labour is a long way ahead of the opposition Conservatives.
But Tory leader William Hague has insisted his party is "ready to win" despite its poor performance in the polls.
Mr Hague said it was "very arrogant for people to tell them [the voters] the results have already been decided", adding that he was confident the Tories would win the election on their policies.
Liberal Democrat Charles Kennedy expressed his relief that the long-awaited campaign was finally under way and declared his party was "raring to go".
Both leaders had been out on the stump for some hours before Mr Blair's visit to Buckingham Palace.
Mr Kennedy visited a London school with his home affairs spokesman, Simon Hughes, before making arrangements to launch his own party's election offensive.
And Mr Hague told a Save the Pound rally that he wanted to "give you back some of the money stolen from you in stealth taxes".
The campaign themes of the three main parties are expected to follow traditional lines.
Reform of, and greater spending on, public services will be central to Mr Blair's fight for re-election, while the Tories are offering tax cuts and will highlight "stealth tax" rises imposed by the government over the past four years.
The Lib Dems will stick to their guns and, alone of the main political parties, once again argue the case for increased taxes to fund improved public services. Personal freedom will also be a key Lib Dem campaign theme.
Welsh, Scots and Green challenges
In Glasgow, SNP leader John Swinney - whose party forms the official opposition in the devolved Scottish Parliament - said: "We are the party for Scotland and know the importance of sending a strong Scottish voice to Westminster where vital decisions will be made involving Scotland."
In Cardiff, Plaid Cymru leader Ieuan Wyn Jones said his party would mount "the most concerted challenge ever" to Labour in Wales.
"The people of Wales are angered that Tony Blair and New Labour have turned their backs on them."
The Green Party opened its campaign by declaring that it was the "party of social justice".
Principal speaker Margaret Wright said: "The
Greens are the clear choice for voters who want to put people and public service before privatisation and profit."