What could a hung Parliament mean for each of the parties?
There are 650 seats in the House of Commons. That means a party needs 326 MPs for a majority. Without it, parties have two options:
- Try to govern with a minority in ad hoc agreements with smaller parties
- Or parties can seek to form a coalition with another party.
This is what the parties are currently saying:
The Conservatives currently have 302 seats.
They are predicted to end up with 306, the largest party in Parliament but 20 short of the 326 needed to secure a majority.
David Cameron has said Labour have lost the "mandate" to govern after trailing the Tories in terms of seats and votes won, describing them as the "outgoing" government.
He has outlined the terms of what he said was a "big, open and comprehensive offer" to the Lib Dems which he hoped could form the basis of a "strong government".
The BBC's Nick Robinson said it was not clear whether this amounted to an offer of a coalition - including possible cabinet posts - but said Conservative sources had not ruled this out.
As part of this offer, Mr Cameron offered to hold a "committee of inquiry" into future reform of the voting system. Proportional representation is a key policy objective of the Lib Dems, so it remains to be seen whether the Tory leader's offer goes far enough.
Mr Cameron said he believed the two parties could find common ground on certain areas such as reform of the tax system, increased money for education and building a low-carbon economy.
However, he said any agreement would centre on the urgent need to tackle the deficit and said the Conservatives would expect to implement key manifesto commitments - including in areas such as Europe, immigration and defence.
Mr Cameron said he hoped discussions could be concluded "quickly" in the "national interest".
Nick Robinson said a combination of the Conservatives and the Lib Dems would be the only potential "stable majority available" but there were plenty of hurdles still to overcome.
However, the BBC has been told Conservative MPs will not countenance any commitment to changing the voting system.
Senior party figures say any attempt by Labour to try and form a coalition with the Lib Dems would be tantamount to a "coalition of the defeated" and be unacceptable to the public.
If no Lib Dem deal is agreed, the Tories may try to form a minority administration and work with smaller parties such as Northern Ireland's Democratic Unionists.
The BBC's Nick Robinson said an alignment with the Democratic Unionists and the one independent Unionist elected would create an estimated bloc of 315 seats.
The Lib Dems currently have 57 seats.
During the campaign, leader Nick Clegg said the party which won the most seats and votes would have the "mandate" to try and seek to form a government.
Speaking on Friday morning, he said that he "stuck by" that position and that the Conservatives had the first right to seek to form a government in the "national interest".
He has yet to respond to Mr Cameron's overture but Lib Dem sources have said Mr Cameron's proposal was "interesting".
In any discussion they say a priority for them is ensuring financial stability going forward.
Several weeks ago, Mr Clegg called a meeting of his MPs for Saturday to discuss their next step and this meeting will be used to debate the Tory offer.
Mr Clegg needs the support of the parliamentary party and the party's ruling federal executive in order to strike any deal.
However, unless three-quarters of these bodies back him, a special conference will have to be convened to decide whether to proceed with a deal.
At one stage, the Lib Dems suggested voting reform in the future would be a pre-condition of any agreement with another party but have since seemed to go back on this.
Labour currently have 256 seats and are predicted to end up with 261.
The constitutional convention states that - in the event of no party winning a majority - the sitting prime minister remains in place until he decides he cannot form a government and chooses to resign.
Gordon Brown has sought to reassure the public about the situation, acknowledging that people are likely to be worried about the current "uncertainty".
In a statement outside No 10, Mr Brown said the current government would continue to discharge its key economic duties for the time being and he would work to "resolve" the current political deadlock for "the good of the country".
He said he "completely respected" the Liberal Democrats' position that the Conservatives should have first right to try and form a government.
The Conservatives and the Lib Dems should have all the time they need to hold discussions about a possible agreement, he added.
Should these talks not lead anywhere, he said he was "prepared" to hold discussions with the Lib Dems about "measures" which could form the basis of a deal with Labour - such as reform to the voting system and support for the economic recovery.
Mr Brown earlier asked Cabinet Secretary Sir Gus O'Donnell to arrange for the Civil Service to provide support on request to parties engaged in discussions on the formation of government.
Senior Labour figures have backed a possible coalition deal with the Lib Dems but, on current predictions, it would not be enough to secure a working majority in Parliament.
Should they reach any agreement, the two parties are likely to end up with 317 - nine seats short of the 326 required to command a majority.
With eight constituencies still to declare, Labour sources say it is important the combined tally of Labour and Lib Dem is higher than the Tories for any deal to have "legitimacy".
Nick Robinson said that if one included the three SDLP MPs and the one Alliance MP - likely to be sympathetic - there would be a slightly larger possible alignment.
The Scottish National Party (SNP) and Welsh nationalists Plaid Cymru, who have a formal alliance, currently have nine MPs.
SNP leader Alex Salmond told the BBC he had ruled out any deal with the Conservatives but suggested the door was open to possible discussions with a Labour-led grouping.
Asked what the SNP would want in return for offering their support, he said they would not be "showing their hand" ahead of any potential negotiations.
He has previously said a hung Parliament would be good for the two parties as they would be able to get the "best possible deals" for Scotland and Wales.