Addressing a May Day rally in Colombo, Rajapaksa, who leads Joint Opposition camp, said that Trincomalee's oil tanks preserved as national asset by successive governments since independence are to be sold.
"This is a government which auctions all national assets," Rajapaksa said.
The reference was on the proposed deal to start a joint venture with India to utilize the World War II time tanks.
Workers of state-run petroleum company went on a strike last week over the issue. They, however, ended their strike after an assurance from Prime Minister Ranil Wickremesinghe.
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One of Rajapaksa' main backers Wimal Weerawansa, a former minister, called for raising of black flags during Modi's visit to Lanka.
"They are trying to sell our country to India," he told the rally.
Modi is due to visit Lanka on May 12 to attend the UN Vesak Day, which commemorates the birth, enlightenment, and death of Buddha.
Modi would only attend the UN 'Vesak Day' celebrations and no bilateral agreements will be signed during his visit to Lanka, President Sirisena has said.