Some 5,000 students have protested in the Ukrainian capital Kiev against what they allege were falsified results from Sunday's presidential election.
Students called for election officials to be punished
There was also an angry debate in the Ukrainian parliament, with calls for a recount and for key officials to quit.
Prime Minister Viktor Yanukovych will face a run-off with his rival Viktor Yushchenko, as they are in a dead heat with nearly all votes counted.
Students held banners saying "Yes Yushchenko!" and "No falsification!"
The Central Electoral Commission had still not announced the final results on Tuesday, two days after the controversial election.
According to the latest results made public, with 97% of the votes counted, Mr Yanukovych was only marginally ahead of Mr Yushchenko - with 39.8% against 39.2%.
The election failed to meet international standards, Western poll observers say.
And in Kiev there is a very widespread conviction that the published results from the first round were not accurate, the BBC's Steven Eke reports.
Anger in parliament
Mr Yushchenko is seen as more reformist and pro-Western than Mr Yanukovych, who was backed by outgoing President Leonid Kuchma. They will face each other again in a run-off on 21 November.
An opposition representative to the electoral commission, lawyer Yury Klitchkovsky, said the actual results put Mr Yushchenko more than 340,000 votes ahead of Mr
Observers criticised numerous election shortcomings
In parliament, opposition leader and Yushchenko ally Yulia Tymoshenko said "the election fraud did not work".
She called for "a dedicated battle against the criminals in power".
At the demonstration, speakers demanded the reinstatement of students expelled from universities for campaigning for the opposition.
They also called for an end to alleged harassment of student activists by the police and security services, and a full investigation into the violations reported across Ukraine during the first round of voting.
Later on a recording was played: made secretly inside a university, an unnamed woman is apparently heard telling students where they should tick their ballot papers, and that they should be dropped unfolded into the ballot box.
Ukraine's 37 million voters were asked to choose between 20 candidates contesting the election.
The Organisation for Security and Co-operation in Europe (OSCE) said the election was "a step backwards from the 2002 [parliamentary] elections".
"We have to conclude that this election did not meet a considerable number of... European standards for democratic elections," an OSCE statement said.
The OSCE listed several concerns about the electoral process, including:
- Bias by the state media
- Interference by the state administration in favour of Prime Minister Yanukovych
- Disruption or obstruction of opposition campaign events by the state authorities
- Inadequacies in the Central Electoral Commission's handling of complaints.