Despite agreeing that the standoff must be resolved peacefully, opposition leader Viktor Yushchenko gave the Kremlin-backed Prime Minister Viktor Yanukovich, who won the controversial poll, a few days to come up with a solution or face action.
"We will allow only a few days for the negotiation process. If Yanukovich wants to drag things out, we will take active measures," he told supporters after the three-hour talks.
Yushchenko insisted on a 12 December re-run of the polls, in which the official ballot count handed victory to his opponent by a margin of less than 3% amid overwhelming allegations of fraud.
"The prime minister is proposing things that will take Ukraine further away from the resolution of the political crisis," he told a crowd of tens of thousands in Independence Square following the talks.
"The prime minister is proposing things that will take Ukraine further away from the resolution of the political crisis"
"We insist on the following: the main precondition for the talks is the holding of new elections for the president of Ukraine," Yushchenko added.
The pair shook hands after the negotiations, held in the presence of European Union and Russian mediators, but the tensions of recent days was palpable.
After they left the talks, outgoing President Leonid Kuchma read a joint statement signed by them both.
"We stand against any use of force that might lead to an escalation of conflict and bloodshed," the statement read.
"To ensure the negotiation process, the parties established a working group in order to elaborate a joint decision on the settlement of the political crisis," it went on.
Both candidates for Kuchma's succession, who agreed to the talks amid an escalation of rhetoric between Russia and Western nations, stood silently as the president read out the agreement and left without speaking to reporters.
A flurry of closed-door negotiations between Ukraine's top players and European dignitaries highlighted the standoff's fifth day that culminated with Yanukovich and Yushchenko meeting for the first time in a suburban Kiev retreat.
Talks to end a crisis that has locked Moscow and the West in confrontation was also attended by President Leonid Kuchma, European Union's foreign policy chief Javier Solana, Polish President Aleksander Kwasniewski, Russian parliament speaker Boris Gryzlov and Lithuanian President Valdas Adamkus.
"The life of every individual is dear to me and I do not need any power if this provokes bloodshed"
"I would like to stress once again that only a legal compromise can help to resolve this very complicated situation which arose in Ukraine after the second round of the presidential election" on Sunday, Kuchma said as he opened the meeting.
The roundtable was agreed amid growing signs that the seat of power was relenting - even as Russia again vented its fury at what it said was unlawful Western pressure being applied against its most important eastern European ally.
Ukraine's national television stations that had throughout the week provided mostly positive coverage of Prime Minister Yanukovich and avoided broadcasting news of the mass demonstrations gripping the country began to feature the opposition in its news.
Supreme court to meet
Meanwhile the supreme court was expected to hear opposition claims on Monday that the state had fixed last weekend's vote in favour of Yanukovich. Analysts said that its justices have shown independence in the past and could possibly back Yushchenko's position.
Russian officials say the West is
meddling in Ukrainian affairs
The prime minister entered the meeting after first telling a sea of supporters assembled outside Kiev's main train station that he would never allow the opposition demonstrators to stage an "anti-constitutional coup".
But he also urged his supporters to keep calm.
"The life of every individual is dear to me and I do not need any power if this provokes bloodshed," he was quoted as telling a Kiev rally by Interfax.
Moscow greeted Europe's direct intervention in the crisis with open fury and dismay.
Russian Foreign Minister Sergei Lavrov said in Moscow that Europe was trying to pull its ally into the Western camp by dangerously and illegally fomenting opposition protests.
"We are alarmed by attempts by certain governments to steer the situation in Ukraine away from a legal path," Lavrov said.
"We are alarmed by attempts by certain governments to steer the situation in Ukraine away from a legal path"
Russian foreign minister
"Especially when certain European capitals are declaring that they do not recognise the elections and that Ukraine has to be with the West," he added.
The comments were some of the harshest from Russia because they directly confirmed that Moscow was interested in seeing its own allies rule the strategic former Soviet republic.
The Slavic nation of 48 million has served as a buffer between Russia and the expanding European Union - which Moscow is viewing with growing mistrust - and remains its key trading partner, particularly in the military sphere.
Meanwhile Yushchenko supporters who throughout the week kept up a non-stop vigil on key Kiev squares moved directly against the government's seat of power en masse for the first time.
Yushchenko has called for people
to protest peacefully
Tens of thousands of chanting, cheering and singing protesters blocked the presidency and the government building in a human chain of opposition orange that kept all from entering or leaving the buildings.
Some officials tried to sneak in through back doors and secret entrances only to be turned away by the crowds.
The police presence was light and some quietly said they were following orders but were with the people in their hearts.
"Of course I am with them, the police are with the people," said Vadim, a policeman in his late 20s, who was standing with the protesters near the building.
His comments came as some 25,000 Yanukovich supporters gathered at a railway station before their planned march in the direction of their opposition rivals who have made the capital's central Independence Square their home.