Gordon Brown, David Cameron and Nick Clegg are making their final day push for votes, as polls suggest the election result is on a knife edge.
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”.
Liberal Democrat leader Nick Clegg said this was “a once-in-a-generation opportunity to do things differently”.
In other election developments on Wednesday:
In Scotland, the SNP, Lib Dems, Labour and Tories seek to rally supporters and win over voters
In Wales Plaid Cymru leader Ieuan Wyn Jones, Welsh Lib Dem leader Kirsty Williams, Welsh Secretary Peter Hain and Tory leader David Cameron are all visiting target seats
Gordon Brown tells Labour voters thinking of voting tactically it is important his party gets as many votes as possible
David Cameron says he is “going all out” to win , after an all-night campaign
Nick Clegg says only his party can deliver “real change” and it is “time to make a choice”
Other parties seeking votes and hoping to make a breakthrough on the last day of campaigning are the Green Party, UKIP and the BNP.
The parties argue over whether electoral reform is needed to rebuild trust in Parliament
Mr Cameron campaigned overnight, talking to shift workers across the north of England, and is about to rally supporters at his final campaign event in Bristol.
He told GMTV it was the “most important election in a generation” and he had campaigned all night because he “didn’t want to waste any hours on the last day and a bit”.
The signs are that millions of voters are agonising, weighing up in their minds the two most powerful messages in politics – time for a change and don’t risk it.
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.”
After a day in which cabinet ministers Ed Balls and Peter Hain urged tactical voting to keep the Tories out – Mr Brown, the prime minister, told a phone-in on BBC Radio 5 live that he wanted all Labour supporters to vote Labour.
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.”
I’m a fighter, I don’t give up
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.”
In a speech in Bradford, he said he was “determined” and “resolute” and told supporters: “This is not a Conservative moment.”
After visiting a haulage firm in Carlisle, Mr Brown’s campaign will culminate with a rally in the south of Scotland.
Meanwhile Nick Clegg, who is hoping his party can make the breakthrough from their traditional third place, addressed a rally in Eastbourne, before heading north to Durham. He will end his campaign with a rally later in Sheffield.
He has been visiting seats he would have considered unwinnable a month ago – in Durham two seats were won with large Labour majorities in 2005 – and he is urging disaffected Labour supporters to come over to him.
He told students in Durham that “nothing will really change” if either Gordon Brown or David Cameron make it to Downing Street.
Urging young voters to “make your voices heard”, he said: “If you give us a chance, if you trust us with your vote, I promise I will do everything I can to make things better for good and deliver the fair Britain you want.”
In a series of interviews with the BBC’s Six O’Clock News, Mr Brown said he had the “judgement and values” to make the big decisions, Mr Cameron said only a “decisive” vote for the Tories would move the country in a “new direction” while Mr Clegg urged voters to do “something different” this time.
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%.
A Comres poll for ITV News and the Independent suggests there has been no change since its last survey on Monday. The survey has the Conservatives on 37%, Labour on 29% and the Lib Dems on 26%.
Labour are finished, while the Tories are arrogantly saying they can rule with no Scots MPs
Alex Salmond, SNP leader
As polls continue to suggest the election will result in a hung parliament, Green leader Caroline Lucas – who hopes to become her party’s first MP – told the BBC there were “pretty exciting days ahead” as the Greens would get “that bit more influence”.
“It’s incredibly important that you’ve got people that will be there day-in, day-out, saying the environment is a crucial issue, doing good action on it, but also showing that it benefits people,” she said.
Plaid Cymru’s leader Ieuan Wyn Jones, campaigning in Llanelli, said whoever won power, Wales was facing cuts: “By voting Plaid we can defend Wales better against those cuts and the greater the vote for Plaid, the better the deal we can get for the people of Wales.”
And SNP leader Alex Salmond said only his party could protect Scotland from the “worst impact of a Tory or Tory-led government”: “Labour are finished, while the Tories are arrogantly saying they can rule with no Scots MPs and the Lib Dems are ready to do a deal with David Cameron.”