Ukrainian President Viktor Yushchenko's decree to dissolve parliament came into force on Tuesday, putting him in direct confrontation with his prime minister and rival Viktor Yanukovich.
Yushchenko is locked in a long-running power struggle with Yanukovich and, on signing the decree on Monday, he said he was acting to "preserve the state."
The decree published in the official gazette also set a parliamentary election for May 27.
Even if all sides agree to take part in the election, it could produce more of the political stalemate that has plagued Ukraine since the last parliamentary election barely a year ago.
Yanukovich had urged the country's pro-Western president not to publish the decree in the official gazette after which it automatically comes into force. His backers in parliament said the move was akin to the start of a coup.
The tough stances adopted by both sides raised tensions in the former Soviet republic 2-1/2 years after the so-called "Orange Revolution," when peaceful mass protests forced out the political establishment that had held power for years.
The immediate prospect was that the pro-Russian Yanukovich might refuse to accept the dissolution, tipping the ex-Soviet state into a constitutional crisis and increasing the risk of clashes in the streets between rival protesters.
About 2,000 Yanukovich supporters held protests near the parliament and around Independence Square, the focus for the "Orange Revolution."
Lawmakers from Yanukovich's coalition in parliament have written to the Constitutional Court asking it to rule on whether Yushchenko was acting legally by dissolving parliament, Interfax news agency reported on Tuesday.
Fear of Confrontation
Yanukovich has not said explicitly if he will defy the dissolution but in a late-night cabinet meeting on Monday he hinted at possible confrontations.
"I will not speak aloud of a third option. That would boost tension greatly ... The president would be fully responsible for the heavy burden."
Parliament will continue sitting until the election. It was in session on Tuesday and pro-Yanukovich lawmakers vowed to hold a round-the-clock vigil in the chamber.
In Washington, U.S. State Department spokesman Sean McCormack said in a statement: "The United States calls on all Ukrainian political leaders to take full responsibility for their supporters' actions and to maintain calm."
Yushchenko said on live television late on Monday it was his duty to dissolve parliament because it had violated the constitution.
"My actions were prompted by a crucial need to preserve the the state, its sovereignty and territorial integrity," he said.
The president, who advocates future European Union and NATO membership, said Ukraine's affairs were "under control."
Though it has lost some ground this week, the Ukrainian stock market is up more than 60% this year, making it among the world's best performers. The deepening political crisis could eat into those gains, analysts said on Tuesday.
"The president's decision further enhances nationwide political uncertainty," Renaissance Capital said in a research note.
Yushchenko beat Yanukovich in the re-run of the rigged 2004 election that triggered the "Orange Revolution" protests. His powers have been cut since by constitutional change and his popularity has sunk after liberals accused him of indecision.
Opinion polls suggest parliament would once again be split between Yanukovich's supporters and his opponents.