(Reuters) - Pakistan's ousted prime minister Imran Khan disbanded a protest march by supporters on Thursday after clashes with police outside parliament, but threatened that they would return unless an election was called within six days.

Khan had rallied thousands of supporters to Islamabad, with plans to occupy sensitive parts of the capital until Prime Minister Shehbaz Sharif gave in to his demand for new polls, but Khan told his followers on Thursday morning to step back, while delivering a fresh ultimatum.

"I'm giving you six days. You announce elections in six days," Khan said from atop a truck after he and thousands of his supporters reached the city.

He said parliament should be dissolved to hold elections in June, and warned the government that he will lead a march on the capital again if it didn't meet his demands.

Khan's attempt to destabilise Sharif's month-old coalition government risks fuelling tensions during an economic crisis that has forced Pakistan to seek urgent help from the International Monetary Fund.

The government convened a joint session of parliament on Thursday to discuss the economic crisis following talks with IMF officials in Doha a day earlier.

The IMF said that considerable progress had been made, but emphasized the urgent need for Pakistan to remove fuel and energy subsidies.