Tory leader David Cameron and Liberal Democrat leader Nick Clegg held face-to-face talks tonight as they continued their attempts to thrash out a power-sharing deal.
A senior Conservative Party spokeswoman said the talks were "constructive and amicable".
The meeting took place at Admiralty House in Whitehall and lasted for 70 minutes. Further negotiations between the two parties are planned tomorrow.
The spokeswoman said: "David Cameron and Nick Clegg met alone this evening at Admiralty House. The meeting lasted 70 minutes and was constructive and amicable.
"The negotiating teams will meet as planned tomorrow at 11am to continue discussions."
A Lib Dem spokesman also described the discussions as "constructive and amicable".
The first face-to-face talks between the leaders since Thursday's General Election came after Mr Clegg secured his party's endorsement for his decision to enter talks with the Conservatives over possible co-operation in forming a new government.
Three separate meetings of senior frontbenchers, the 57 Lib Dem MPs and the party's federal executive each gave their "full and complete" backing to the leader's approach.
Tomorrow's meeting at the Cabinet Office will bring together the Tory team of George Osborne, William Hague, Oliver Letwin and Mr Cameron's chief of staff, Ed Llewellyn, with Lib Dem negotiators David Laws, Chris Huhne, Danny Alexander and Andrew Stunell for the second time, following initial talks on Friday evening.
Tory sources indicated that no conclusion to the discussions was expected before Monday at the earliest, while Mr Laws refused to put an "artificial timescale" on the process.
Meanwhile, Gordon Brown - who remains Prime Minister until the impasse caused by the inconclusive election result is resolved - remains ready to talk to the Lib Dems about a possible alliance if discussions with Tories run into the sand.
There were apparent signs today that Mr Clegg may be keeping his options open in negotiations with Tories on the crucial Lib Dem priority of voting reform.
The Lib Dems were warned by a senior Tory not to "hold the country to ransom" with demands for proportional representation (PR), which is thought likely to be a deal-breaker if Mr Clegg makes it an absolute condition of any co-operation.
Shadow defence secretary Liam Fox said: "It would seem to me very strange in an election that was dominated by the economy... if the government of the UK was held to ransom over an issue that the voters did not see as their priority."
Mr Clegg has previously described a fairer voting system as an an "absolute pre-condition" for the renewal of Britain's politics. But as he arrived for today's meetings, he said his negotiating strategy would be guided by four Lib Dem objectives including "fundamental political reform of our political system".
And addressing around 1,000 demonstrators calling for PR outside the Smith Square meeting, he said he was driven by the goal of "reforming politics", carefully avoiding any mention of the voting system.
Mr Clegg was cheered as spoke to protesters, including musician Billy Bragg, by megaphone, though he was also faced with calls of "Don't sell out, Nick!"
"Take it from me, reforming politics is one of the reasons I went into politics," he told the crowd.
"I've campaigned for a better, more open, more transparent, new politics, every single day of this General Election campaign. I genuinely believe it is in the national interest."
Mr Brown has offered immediate legislation on a referendum on electoral reform in return for a deal with Lib Dems to keep him in Downing Street. And senior minister Peter Hain has made clear that this could go beyond the Alternative Vote (AV) system previously favoured by Labour.
Asked whether Lib Dems would insist on PR as part of any deal with Tories, Mr Laws said only that it was "a matter which would have to be discussed".
Before making any move which might impact on the Lib Dems' political independence, Mr Clegg must negotiate a "triple lock" which requires him to secure the support of the party's MPs and federal executive, and may involve a special conference or a postal ballot of members.
But sources indicated that there was no vote on his plans at today's meetings, which instead saw him briefing MPs on the positions outlined to him by Mr Brown and Mr Cameron in brief phone conversations last night. They denied suggestions that Mr Cameron had offered the posts of home secretary, chief secretary to the Treasury and transport secretary to Lib Dems in a coalition cabinet.
BBC reports of an angry exchange with Mr Brown in which the PM launched a tirade of threats against Mr Clegg were categorically denied by both Lib Dems and Labour today.
Speaking outside Lib Dem headquarters in Cowley Street, Mr Laws characterised today's meetings as "very, very positive and constructive" and said both frontbenchers and MPs had "endorsed in full and completely the strategy laid out by Nick Clegg over the last 24 hours".
He added: "We are determined to put the national interest before party advantage and to play our part in delivering the stable and good government the people of this country are entitled to expect...
"We understand the pressures that there are to make a decision on these matters as soon as possible and we are keen for an early conclusion to these issues. But people will also understand that we are keen to make sure that we make the right long-term decisions for the people of this country."
Mr Clegg joined Mr Cameron and Mr Brown in taking part in a commemoration at the Cenotaph in Whitehall of the 65th anniversary of VE Day. The ceremony had been expected to be the first public engagement of the new Prime Minister after Thursday's election.
Bassetlaw MP John Mann today became the first Labour figure since the election to call on Mr Brown to stand down as leader, arguing that he could not secure a credible pact with the Liberal Democrats.
Labour was dismissive of a call from Scottish National Party leader Alex Salmond for a "progressive alternative" alliance involving the SNP and Plaid Cymru.
And there were the first stirrings of dissent in the Tory ranks, with former chairman Lord Tebbit publishing an article calling for a ballot of the membership on any compact with Liberal Democrats.
Mr Clegg and Mr Brown this evening spoke by telephone, the Liberal Democrats revealed.
A party spokesman said: "At the request of the Prime Minister, Nick Clegg took a call from Gordon Brown this evening. The conversation was amicable.
"The Liberal Democrats will continue with the approach which Nick Clegg has set out and which was endorsed today by the parliamentary party and the party's federal executive."