The contest, announced after weeks of wrangling, will be the only time Conservative PM David Cameron and Labour’s Ed Miliband face one another in a debate before polling day reports BBC.
The Lib Dems, SNP, UKIP, Plaid Cymru and the Greens will also take part.
Norman Smith, BBC assistant political editor, said the stakes were in a way highest for Mr Cameron and Mr Miliband.
He said the smaller parties had less to lose, but tonight’s debate would be a “visible symbolic demonstration of the death of two-party politics.”
There had been doubt over whether a debate between leaders – first held in 2010 when then PM Gordon Brown, Mr Cameron and Mr Clegg participated in three events – would be repeated before the 7 May poll.
Mr Cameron had rejected the initial proposals because they did not include the Greens, and also said any debate should take place after the start of the campaign on 30 March.
The final schedule also included a live question and answer programme featuring David Cameron and Ed Miliband appearing separately, shown on on Channel 4 and Sky News last week, and a BBC debate involving opposition party leaders, moderated by David Dimbleby on 16 April.
There will also be a special Question Time on BBC One, a week before polling day, with Mr Cameron, Mr Miliband and Lib Dem leader Nick Clegg appearing separately to answer questions from a studio audience.
The Democratic Unionist Party, which has eight MPs, has strongly criticised its exclusion from the programme.