Polls have opened to millions of voters in the most uncertain General Election for decades, with no party on course to emerge a clear winner.
Last-ditch appeals by party leaders did nothing to break the opinion poll deadlock and left the country facing the prospect of another hung parliament.
But with millions apparently still undecided or open to changing their minds, the likely new Westminster balance of power remained highly unclear.
Among the last set of polls, three showed the main parties level pegging, three had the Conservatives in front by a single point and one gave Labour a two-point advantage.
David Cameron said the way voters cast their ballots would "define a generation" and appealed for more time to build a better Britain, warning a Labour government would be "held to ransom" by Scottish nationalists.
But Ed Miliband accused him of hiding the truth about deep spending cuts that posed a "real and present danger" to families' finances and urged people to bring an end to "five years of unfairness, five years of failure".
In one of the biggest pre-election polls, a YouGov survey of 10,000 voters for The Sun had the main parties on 34% each - but with a significant 17% saying they were yet to make up their minds - a figure put as high as 25% in a ComRes poll for ITV and the Daily Mail.
In the past such a tie would have been enough to propel Ed Miliband into Downing Street but an SNP surge in Scotland threatens to rob Labour of dozens of its traditional strongholds north of the border and of the chance to govern alone.
A YouGov poll in Scotland for The Times show Nicola Sturgeon's party - with which Mr Miliband has ruled out any formal deal - enjoying 48% of support to Labour's 28%, putting several key figures including Scottish Labour leader Jim Murphy in peril of losing their seats.
Ms Sturgeon said her party was "within touching distance" of winning a majority of Scottish seats at Westminster for the first time and being able to make sure "the voice of Scotland is going to be heard more loudly at Westminster than it has ever been heard before".
She has appealed to Labour to join forces to "lock out" the Conservatives but warned her MPs would vote down a future Labour budget if it failed to end "Tory austerity" - a threat that has been seized on by the Tories as a central theme of its campaign.
Lib Dems in fightback
Nick Clegg - who faces a fight to hold on to his own Sheffield Hallam seat - urged voters to stick with the Liberal Democrats as the only party able to provide a "stable" influence to a Tory or Labour administration.
He said his party's performance will be the "surprise story" of polling day, dismissing predictions of an electoral mauling that has left key figures such as cabinet minister Danny Alexander vulnerable to a collapse in support after five years of governing in coalition with the Conservatives.
Ukip leader Nigel Farage predicted many undecideds would swing behind the Eurosceptic party as it seeks to translate regular third places in national polls into an influential Commons presence in any post-election negotiations.
"We have a feeling there are lots of people out there who are shy Ukippers who don't tell the opinion pollsters how they will vote," he told a rally - adding he was looking forward to the established parties waking up tomorrow with a "huge hangover".
The Green Party will also hope to increase its parliamentary presence, heavily targeting three seats in a push to underline the increasingly fractured political make-up of the electorate.
Posting on Facebook hours before voting began, Mr Cameron said: "Amid all the confusion and commentary, my message is simple and clear: Britain has the chance of a strong, stable government - but only if you vote Conservative. All other options will end in chaos.
Last pitches to voters
"So as you enter the voting booth, remember these simple things: you can stop Ed Miliband being held to ransom by Nicola Sturgeon and the SNP. You can ensure strong and stable government. You can secure our economy and the Union. You can ensure I am back at work as your prime minister on Friday.
"The country's future depends on the choice you make."
In a final plea to people to vote Labour, Mr Miliband will say: "If you do that today, then tomorrow you won't have to wake up to the news that five years of the Tories has turned into a Tory decade - a decade where only the privileged few will do well, where there will be one rule for a few and another for everyone else.
"So the stakes are high in these crucial hours."
He said the Tories planned "the most extreme spending cuts of any political party for a generation" that posed "a clear and present danger to the family finances of working people".
Polling stations will be open until 10pm in what will be the busiest General Election day since 1979, with nearly 10,000 council seats also up for grabs.
There are contests for 290 councils and six mayors in England.