EU foreign policy chief Javier Solana has returned to Ukraine for talks on the country's political crisis.
Yushchenko's supporters are still on the streets of the capital, Kiev
He and other foreign envoys are due to meet outgoing President Leonid Kuchma in an attempt to smooth the way for a re-run of disputed elections.
Mr Kuchma has conditionally accepted opposition calls for electoral reform, but says parliament must simultaneously agree to curb presidential powers.
On Friday the supreme court backed claims that the election was tainted.
Russia's President Vladimir Putin has said he is ready to work with whoever is elected in the 26 December re-run.
The opposition's Viktor Yushchenko is running against Moscow's favoured candidate, current Prime Minister Viktor Yanukovych.
But Mr Putin warned Mr Yushchenko was seeking power at the risk of Western interference.
"Only the people of any country - and this includes Ukraine - can decide their fate," he said during a visit to Turkey.
"One can play the role of mediator but one must not meddle and apply pressure."
The Organisation for Security and Co-operation in Europe (OSCE) wants to send more monitors to the Ukraine for the new poll to ensure it is free and fair.
Mr Kuchma asked Mr Solana and Jan Kubis, OSCE Secretary-General, to travel to Kiev to continue the mediation process.
A Brussels-based spokeswoman for Mr Solana told AFP news agency: "Our hope is mainly to maintain the process as set up by the [Ukrainian] supreme court.
Neither Mr Solana nor the EU as whole has expressed a preference for either candidate, the spokeswoman added, insisting that the "process" of the re-run election is crucial to its legitimacy.
Polish President Aleksander Kwasniewski, Lithuanian President Valdas Adamkus and Russian envoy Boris Gryzlov will also take part in talks due to begin at 2000 local time (1800 GMT).
The continuing standoff over Mr Kuchma's proposed constitutional reforms is expected to top the agenda.
The president said on Monday he still would not accept election law reforms demanded by the opposition unless they agreed to constitutional reforms, which would weaken the presidential role.
"I am ready for further steps to lift this unwarranted tension, so I propose once again looking at the whole packet of bills to change the election law and on changing the constitution," he said at a government meeting.
Mr Kuchma also said he would not dismiss the government, which lost a parliamentary no-confidence vote, until the constitutional reform was approved.
The BBC's Mike Donkin in Kiev says Mr Kuchma's offer would mean that if Mr Yushchenko did win in three weeks' time he would have the top post, but real power would be in the hands of Mr Yanukovych as prime minister.
Yulia Tymoshenko, an associate of Mr Yushchenko, has already said the deal is unacceptable. She is expected to be the opposition candidate for prime minister.
Demonstrators in the streets of Kiev are saying they will keep up their blockade of government buildings until they get reforms to stop the irregularities which marred the original 21 November election.
But witnesses say protesters have started letting some civil servants into their offices.
The opposition has refused to vote on presidential powers and electoral reform in the same package. It wants constitutional changes considered separately at a later date.
Foreign ministers from the 55-nation OSCE are meeting in the Bulgarian capital, Sofia.
They are due to decide whether to nearly double the number of poll monitors to 1,000.