United National Party leader Ranil Wickremesinghe and supporters set off from the southernmost tip of the island for the capital Colombo, 170km north, where they will arrive for a rally on July 12.

Kumaratunga's government was reduced to a minority after Marxist allies quit the government last month to protest a deal to share billions of dollars in international donor aid with ethnic Tamil rebels to rebuild the country after the December 26 tsunamis.

Previously Wickremesinghe said his party, the largest in the 225-seat parliament with 88 seats, would support her government as it implements the tsunami aid plan.

But on Saturday he signalled his backing was not open-ended.

"We are ready for any election," Wickremesinghe told AFP. "The expectation in the country is for a presidential election, but if the government wants to have even an early parliamentary election, we are ready for that."

Kumaratunga holds the powerful post of president and leads the Freedom Party which formed a government with allies after defeating Wickremesinghe in elections in 2004.

The opposition leader also accused the government of scuttling a peace process with Tamil Tiger rebels who have warned that a truce clinched in February 2002, when Wickremesinghe was prime minister, was in danger of collapsing.

"The government has messed up the peace process," Wickreme-singhe said. "The peace process is being turned into a march towards war. I can tell the government only one thing. They just can't deliver."

Kumaratunga accused Wickrem-esinghe of making too many concessions to the Liberation Tigers of Tamil Eelam during six rounds of direct peace talks and used her post as president to call the April 2004 elections.

Despite opposition from allies, Kumaratunga signed the tsunami aid deal with the rebels under prodding by international donors who said it was the only way to ensure help reached people in areas under rebel control, mainly in the east of the island.