Tens of thousands of Ukrainians have marched in a tense Kiev to demonstrate their support for the opposition ahead of next weekend's election.
The streets were a sea of Yushchenko's orange
They waved the orange flags associated with opposition presidential candidate Viktor Yushchenko.
"We demand fair elections," he told the crowd, amid accusations that ballot-rigging could take place.
Mr Yushchenko's staff said 150,000-200,000 people joined the rally. Police put the figure at 30,000.
"Look around, we have beautiful people and a great country. But most are poor and struggling to survive. We need change,"
a student called Ivan told the Reuters news agency. "Yushchenko is our hope."
There was some violence, however, with windows smashed at the central election commission.
The authorities blamed Yushchenko supporters, but the opposition said the incident was staged by government forces.
In the election Mr Yushchenko is pitted against 23 candidates, including the chosen successor of outgoing President Leonid Kuchma, the pro-Moscow Prime Minister Viktor Yanukovych.
The vote is expected to go to a run-off in November between the two men.
Mr Yushchenko, of the centre-right Our Ukraine party, told the crowd "not to stay silent" if they witnessed irregularities during the election.
"The authorities want to buy votes. Their candidate stands
no chance of winning honestly," he said.
"But they cannot buy us off or cheat us or divide us."
He said Ukrainians could enjoy living standards matching those in nearby Poland and the Czech Republic.
Mr Yushchenko has pledged political and
economic reform, and says he will move the country closer to its Western neighbours.
Mr Yanukovych has vowed to improve ties with Russia, take a tighter hold on the economy, and to make Russian an official language.
Western observers have criticised the run-up to the election for what they say is pro-government bias.
The Organisation for Security and Cooperation in Europe has prepared one of its largest election observer missions, comprising 600 officials.
Mr Yushchenko claims he was poisoned during the approach to the poll. Doctors who treated
him in Austria say they have found no evidence for the allegation.