The three leaders are making last-minute appeals to undecided voters
Gordon Brown, David Cameron and Nick Clegg have made their final appeal to the public before voting in Thursday's general election gets under way.
The prime minister said only Labour could take the country "forwards" and would "always be on your side".
Conservative leader Mr Cameron said a victory for his party would bring "hope, optimism and change".
Urging voters to trust their "better instincts", Lib Dem leader Nick Clegg said this election is "our chance".
Polling stations open on Thursday at 0700 BST and will close at 2200 BST.
In other election developments on Wednesday, the last day of campaigning:
Concluding a whirlwind final day of electioneering, the three leaders of the largest Westminster parties all acknowledged that thousands of voters have yet to make up their mind.
Mr Brown told supporters in Dumfries he was the right man to lead Britain through "dangerous and uncertain" economic times and Labour would always have the interests of the British people at heart.
"Give us your support and trust and we will be there every day to support you. We will be steadfast, strong and always on your side," he said.
"Whatever your doubts, whatever your disappointments, if you want to secure a recovery that is fair to all and public services that serve all... I ask you to come home to Labour."
He said Labour's achievements over the past 13 years contrasted with a Conservative Party that had not changed and "still believed in protecting privilege".
"I say let's not go backwards with the Conservative Party, let's go forward with Labour."
At his final campaign rally in Bristol, Mr Cameron urged supporters to "raise their sights" in the last hours before polling day, saying he was fighting to win "with everything we have got".
'Hope and change'
Mr Cameron, who campaigned through the night on Wednesday, said only a Conservative victory could tackle the social and economic challenges facing the country and lift what he said was the "gloom and depression of the Brown years".
"Don't stay at home and let the old guard in," he urged. "Don't vote for the Liberal Democrats and let Gordon Brown in."
A Tory government would provide "hope, optimism and change", Mr Cameron argued, and although there were "difficult decisions" ahead, he pledged to protect the most vulnerable in society and "take people with us".
"I want a government which makes us feel good about Britain again," he said.
Meanwhile Nick Clegg addressed a rally in Eastbourne, before heading north to Durham and ending his campaign with an open-air rally in Sheffield.
As his party strives to make the breakthrough from their traditional third place, he urged disaffected Labour supporters to come over to him.
Describing the election as "a once-in-a-generation opportunity" to break the mould of British politics, he urged people to vote "for the future you want".
"This time is our chance," he said.
After the most "exciting" campaign in a generation, he said the choice faced by voters was clear. "It is a choice between the old politics of the past and the new, different politics of the future," he said.
"I say if you lend us your support I give you this guarantee - I will work tirelessly to deliver the fairness you want."
After what had been one of the most unpredictable elections in modern times, the BBC's Political Editor Nick Robinson said the outcome could not be forecast with any certainty.
The last opinion polls suggest the Conservatives are still some way in the lead but that a hung Parliament still remains a possibility.
A Populus poll for the Times puts the Tories on 37%, up one point on its previous poll last week, Labour up one on 28% and the Liberal Democrats down one on 27%.
Meanwhile, an ICM poll for The Guardian puts the Conservatives up three points on 36%, Labour unchanged on 28% and the Liberal Democrats down two points on 26%.
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.
A ComRes survey for ITV News and The Independent put the Tories unchanged on 37%, Labour down one at 28% and the Liberal Democrats up two at 28%.
The YouGov daily tracker poll for The Sun puts the Conservatives unchanged on 35%, Labour down two points at 28% and the Lib Dems up four points at 28%.
The BBC's head of polling David Cowling said they suggest a nation divided about the outcome.
Ahead of the opening of polls, 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," he said.