Flanked by his entire cabinet, Brown said it was the “least well-kept secret of recent years”.
He said he would seek a “clear” mandate to continue the economic 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”.
‘Road to recovery’
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.
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 paid tribute to the “courage” of the armed forces in Afghanistan and said Labour would “support them in every way”, and would produce a plan to make politics more transparent and accountable.
Mr Brown stressed his ordinary upbringing and 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”.
He criticised 13 years of Labour’s “big government” and said it was time for the Tories’ “big society” instead, adding: “If we win this election, there will be real change.”
Describing it as the “most important general election for a generation”, he urged activists to tell voters: “Let’s get off this road to ruin and instead get on the path to prosperity and progress.”
Cameron welcomes election plan
Lib Dem leader Mr Clegg told reporters earlier on Tuesday 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 were party leaders at the last one in 2005.
It will also be the first campaign to feature live television debates between the three main party leaders.
The whole cabinet is to start campaigning immediately and the shadow cabinet is also poised to fan out across the country.
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.
To secure an overall majority, a party must win at least 326. 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.
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. BBC