Conservative leader Mr Cameron, who campaigned through the night, said he was fighting "for every vote".
Labour Prime Minister Mr Brown said he was "determined" and "resolute" and was "fighting for Britain's future".
But Lib Dem leader Mr Clegg urged people to use their votes to prevent a return to the two-party "stitch-up".
Mr Cameron campaigned overnight, talking to shift workers in Cumbria, Lancashire, Yorkshire and Lincolnshire. He told GMTV it was the "most important election in a generation" and he had campaigned through the night because he "didn't want to waste any hours on the last day and a bit". He said: "I don't want to take anything for granted, it's a very important election, it's a close election and I'm fighting for every vote right down to the wire." Asked about the Tories' narrow poll lead, Mr Cameron said: "I never believed this election was going to be easy. Elections are meant to be a challenge. The British people don't hand you the government of the country on a plate, quite rightly they are making us work for it."
Some polls have suggested that Labour could come third in terms of overall votes - yet still get enough seats to form a government. Mr Brown acknowledged that in some areas the Lib Dems and Tories were battling it out for first place, but said: "People will judge us also on the number of votes we have got, as well as the number of seats." After dealing with a range of listeners' questions on topics including immigration, welfare and mental health funding he said: "I'm a fighter, I don't give up. I'm fighting for Britain's future in my view, I'm fighting because I believe in what I'm doing."
Meanwhile Mr Clegg, is hoping his party can make the breakthrough from their traditional third place. Mr Clegg said in a speech: "It's now time to make a choice, a choice between the politics of the past, the old politics and something new and different for the future. If David Cameron or Gordon Brown get into Number 10, nothing, nothing will really change at all... We cannot let that happen." He added: "We will deliver the real change, the real fairness that people want."
A YouGov daily tracker poll for the Sun, conducted on 3 and 4 May, puts the Conservatives unchanged on 35%, Labour up two points at 30% and the Lib Dems down four at 24%.
Today the people of Greengates Builders Merchants Accrington, Lancashire will be voting.
Every vote matters so make sure you take the time to vote today for the future of our country!