By Ranga Sirilal

COLOMBO, July 26 (Reuters) - Thousands of opposition supporters rallied against Sri Lanka's President Mahinda Rajapaksa and demanded snap elections in the capital Colombo on Thursday.

Protesters chanted "We want a UNP (opposition United National Party) government" and "Save the country from the Rajapaksa brothers". The president's three brothers are also in his administration.

Police said there were 15,000 protesters. The UNP were expecting 100,000.

"We are starting our mass agitation campaign to call for an election," said UNP leader Ranil Wickremesinghe, who narrowly lost the presidency to Rajapaksa in November 2005 when Tamil Tiger rebels boycotted the poll and blocked residents in areas they control from voting.

Rajapaksa's government is now embroiled in a new chapter of the island's two-decade civil war with the rebels. An estimated 4,500 people have been killed since last year.

Wickremesinghe's own party split earlier this year when 17 members, including his deputy, crossed over to the government, helping to give Rajapaksa a long-elusive parliamentary majority.

Protesters said they were fed up with high inflation, which hit 17 percent in June as measured on a 12-month moving average.

"It's really difficult, people can't bear the cost of living," said 38-year-old businessman Ranjith Udumulla, who travelled to attend the protest from the gem-mining town of Ratnapura 40 miles (65 km) southeast of Colombo.

"The government is increasing the burden on the people and using war as the excuse," he added.

Rajapaksa's ruling Sri Lanka Freedom Party staged rallies of its own across the island on Thursday.

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