Viktor Yushchenko narrowly beat his rival, Prime Minister Viktor Yanukovych, in round one on 31 October.

Mr Yushchenko said he believed authorities would try to falsify the vote, but he still expected to win.

"If we do see major electoral fraud, we will bring people on to the street, we will defend our rights," he said.

He said his Our Ukraine party would field 80,000 observers to check for fraud in the country's 33,000 polling stations.

"We affirm that today the Ukrainian voter must be ready to defend his interests as appropriate: on the streets, on the squares in Kiev and in other towns," he said.

Student activists

Mr Yanukovych, prime minister since 2002, was due to hold a major rally in Kiev sports stadium on Friday.

His campaign team has portrayed Mr Yushchenko as an American puppet who will do anything to gain power, including inciting civil unrest.

Mr Yushchenko's followers include a student group called Pora, modelled on the Georgian Kmara and Serbian Otpor organisations, which helped to bring reformers to power after rigged elections in 2003 and 2001 respectively.

Mr Yushchenko came out of the first round of the election with 39.87% of the vote, compared to Mr Yanukovych's 39.32%.

He won about two-thirds of the vote in the capital, Kiev, and was also ahead in the eastern city of Kharkiv, his home region of Sumy, and most western regions of the country.

Mr Yanukovych was ahead in his home region of Donetsk, and most other eastern and southern parts of the country.

Observers from the Organisation for Security and Co-operation in Europe said the election had been marred by severe irregularities.

Among other things, they cited a pro-Yanukovych bias in the state media and interference by the state authorities in favour of the prime minister.

The head of the Central Electoral Commission, Serhiy Kivalov, admitted that there had been many complaints from citizens who found their names missing from the register and were unable to vote.

Dirty tricks

US President George W Bush has sent the head of the Senate Foreign Relations committee, Richard Lugar, to represent him in the country during the election.

The Russian President, Vladimir Putin, who has come out behind Mr Yanukovych, has also sent a trusted ally, Boris Gryzlov.

Mr Yushchenko, prime minister between 1999 and 2001, claims to have been the victim of dirty tricks, including what he alleged was an assassination attempt, when a lorry repeatedly swerved towards his car.

In September, he sought medical treatment in Austria, claiming that he had been poisoned.

His illness, whatever its causes, has caused startling changes to his boyish looks, which used to endear him to some female voters.