The move deepened the country's political turmoil ahead of presidential elections called for October and soured hopes for a quick recovery from a devastating financial crisis.

The main opposition Party of Regions prevented the parliament session from starting because it says the government does not have a comprehensive anti-crisis program.

Prime Minister Yulia Tymoshenko says it does and accuses the opposition of seeking to sabotage her work like "Somali pirates."

Stalling parliament's work by swarming its podium and starting shouting matches and even fist fights has become a trademark ploy in chaotic Ukrainian politics. Tymoshenko's allies in parliament dismissed Thursday's protest as an effort by Party of Regions to generate publicity ahead of presidential elections. Ukraine needs to pass a series of stabilization laws to reduce budget deficit to an estimated 3 percent of GDP in order to receive a second installment of a $16.4 billion loan from the IMF. But the tense rivalry between President Viktor Yushchenko and Tymoshnko and other political forces has stalled those efforts.

Lawmakers passed some of the necessary laws earlier this week, such as raising taxes on alcohol, tobacco and fuel.

But they still needed to vote on measures to cut government spending on pensions and lower state subsidies in the energy sector.

Ukraine is in dire need of the money. The economy is expected to shrink by at least 6 percent this year, according to the IMF. Yushchenko estimated that the economy contracted by up to 30 percent in the first months of this year, largely due to a drop in the global demand for steel, Ukraine's key export commodities, and troubles in the banking sector that slashed lending to enterprises.

The national currency, the hryvna, lost over 40 percent of its value to the dollar since the crisis hit last fall. The number of Ukrainians officially registered as looking for a job jumped from about 600,000 at the end of last year to about 900,000 now.

Experts said the IMF aid was crucial to rescue the economy.

"I think the allocation of the second tranche of the IMF loan needs to be resolved in the next month, otherwise the problems for the Ukrainian economy will just get more and more severe," said Peter Vanhecke, CEO of the Renaissance Capital Ukraine investment bank.

Vanhecke said he was optimistic that the IMF reform package will eventually be implemented and the aid will come through.

"This is so essential that in the end ... people will use their common sense," he said.