Ukraine's constitutional court has begun examining the legality of President Viktor Yushchenko's decree dissolving parliament.
The court has one month to issue its ruling
Prime Minister Viktor Yanukovych, Mr Yushchenko's arch-rival, has refused to obey the decree, which called for an early 27 May election.
Several hundred demonstrators from rival camps have been waving flags and chanting slogans outside the court.
Mr Yushchenko has ruled out any use of force to resolve the crisis.
Last week, five of the court's 18 judges demanded bodyguards, complaining that they had come under "gross pressure" from the prime minister's allies.
Speaking in Strasbourg, before addressing the Council of Europe, Mr Yanukovych said any pressure on the judges would be a "violation of the principles of democracy and the Ukrainian constitution".
The court has one month to consider the case, but the judges have been asked to move faster. A decision requires the consent of 10 judges.
Mr Yushchenko dissolved parliament on 2 April, accusing Mr Yanukovych of trying to usurping power by illegally luring deputies into his parliamentary bloc.
The rivalry between the two men dates back to the 2004 Orange Revolution which swept Mr Yushchenko to power, overturning a Yanukovych election victory which was widely condemned as fraudulent.
Mr Yushchenko and his supporters said Ukraine should aim for membership of the EU and Nato, while Mr Yanukovych pushed for stronger ties with Russia.
Constitutional reforms carried out as a result of the Orange Revolution weakened the president and strengthened parliament, but Mr Yushchenko and Mr Yanukovych have clashed over the details.
Mr Yanukovych asked the court to rule on the legality of Mr Yushchenko's decree.
Mr Yushchenko met the European Commission president, Jose Manuel Barroso, in Brussels on Tuesday.
"This situation requires not only the decision by the constitutional court, but this should be about agreement between the sides... There wasn't even a single discussion about applying any military force for resolving this conflict," Mr Yushchenko said.