By Stephen Mulvey
BBC News, Kiev
The Ukrainian Supreme Court is in a near impossible position.
Yanukovych and Yushchenko: Two Viktors but only one will win
It has been asked to rule on a disputed election under huge pressure from both sides.
On the one hand, hundreds of thousands of demonstrators massing in the centre of the capital, on the other - some of the country's most powerful men.
Wives of Supreme Court judges are reported to have complained that their husbands have been physically threatened to rule in favour of Prime Minister Viktor Yanukovych.
The latest threat from the opposition, meanwhile, is to close down transport links across the country, within 24 hours.
The opposition has filed 11,000 complaints about alleged election irregularities in the regional courts.
Its complaint to the Supreme Court demands that the Central Election Commission's declaration of a victory for Mr Yanukovych should be overturned.
Live broadcasts from the courtroom show 22 judges in maroon robes
hearing arguments from the opposition, the pro-government camp and the
Central Electoral Commission.
Their names were reportedly meant to be kept secret until the last minute to
protect them from outside interference.
Although Ukraine's courts are not generally considered to be fully independent from the government, the Supreme Court itself has taken several decisions that favoured the opposition leader, Viktor Yushchenko, in recent weeks.
It prevented the Central Election Commission from annulling votes in two districts in the Kirovohrad and Cherkasy regions and it upheld a complaint from Mr Yushchenko about campaign videos that he said insulted his dignity.
UKRAINE SUPREME COURT
Considering complaints of poll abuses from both sides
Cannot invalidate whole election, but can annul results in specific districts
Has about 100 judges - up to 40 may sit on Monday
Judges' names remain secret until last minute to prevent pressure on them
Upheld some opposition complaints after first round
A member of parliament who sits on the parliament's legal affairs committee, Mykola Onishchuk, told Ukrainian radio on Monday that the court could issue a ruling before the end of the day.
He said it was possible that the Supreme Court would annul the election results from districts where the worst violations are thought to have taken place, and that this could cause the Central Electoral Commission to say that the outcome of the vote was impossible to determine.
Then either the second round of the vote, or the entire election, might be re-run.
The Yanukovych camp has lodged 7,000 complaints in regional courts, but election monitors indicated that the worst violations occurred in pro-government strongholds in the east of the country.
A BBC Ukrainian Service correspondent said he saw government observers in the west filing numerous complaints of a technical and procedural kind - about the number of ballot boxes in a polling station (two instead of three) or the number of cabins for people to fill in their ballot papers in privacy (four instead of five).
However, the Supreme Court will be only too aware of the danger of inflaming public opinion in the east of the country, where local leaders have already been calling for a referendum on autonomy and threatening to withhold tax revenues.
Invalidating the result in large swathes of the Donetsk and Luhansk regions may be unrealistic, for political rather than legal reasons.
As the Supreme Court considers its verdict, the parliament is also preparing its next step.
Viktor Yanukovych has been touring his eastern heartlands
On Saturday it called for a re-run of the second round of the election and passed a vote of no confidence in the Central Electoral Commission.
Now it wants to make changes to the election law without delay.
These would be designed to prevent some of the violations reported this time from occurring in future, but there could also be an attempt to give the parliament powers to control the Central Electoral Commission.
Currently only the president can dismiss the commission.
Any amendments passed by the parliament would have to be signed by President Leonid Kuchma.
But he, like some of those close to him in parliament, has shown signs of wavering in his support for Prime Minister Viktor Yanukovych.
On Sunday, he praised the resolution passed by parliament the day before but said it had political rather than legal force.
However, he will soon have the chance to give legal force to parliamentary decisions, and the signs are that he may agree to do so.