Bolivians voting in a referendum Sunday overwhelmingly supported President Carlos Mesa's plan for the government to exert greater control over U.S. and other foreign gas companies.

With 66.4 percent of the vote sampled by the government, 87 percent of voters approved a referendum measure that would repeal a 1996 law permitting foreign energy companies to exploit Bolivia's vast and virtually untapped natural gas reserves.

In addition, 92 percent supported a proposal for Bolivia to take back ownership of natural gas at the wellhead, meaning that oil companies would be paid for pumping the oil but would not own it.

The changes would need legislative approval. Final results were not expected before Monday.

"Today the mandate from the people is for a strong state," Mesa said Sunday night, telling Bolivians a new economic era was beginning. Until now, Bolivia has followed U.S.-backed economic policies favoring open markets and little government regulation of business.

Mesa has said he would honor prior contracts reached with U.S. and other foreign gas companies. But with the weight of a referendum behind him, he will have some leverage in asking oil companies to reconsider their contracts.

There are not many precedents for Bolivia's expected move against the gas sector, and Mesa insists he will not replicate Mexico's 1938 expropriation of U.S. oil companies or Chile's 1971 takeover of U.S.-owned copper mines.

But many voters Sunday appeared to believe that they were voting for an immediate seizure of foreign companies, as political parties on both the left and the right are demanding.

Energy industry executives, speaking on condition of anonymity, said they feared radical groups might try to seize gas facilities in coming days and were ratcheting down production.

Bolivia sits atop at least 52.3 trillion cubic feet of certified but largely untouched natural gas reserves, about the size of Canada's reserves.

With support from the United States, Bolivia opened up the gas sector to foreign investors in 1996, and more than $2 billion in foreign direct investment followed.

Mesa wants to ensure that Bolivia has greater control. He proposes reforming the state oil company so that to have it, and not the private sector, would direct development of Bolivia's gas sector. He also proposes raising royalties collected from gas companies to 50 percent from the current 18 percent.

"I'm very optimistic," he said Sunday morning as he toured voting stations in the slums above La Paz, the scene of violence last year that brought down President Gonzalo Sanchez de Lozada and left at least 59 dead.

The violence was sparked by a plan to export natural gas to Southern California through Chile, which seized now-landlocked Bolivia's coastline in 1880. Mesa, a historian who was vice president, assumed the presidency by constitutional succession.

An Aymara Indian casts her vote in Ajllata, Bolivia, during a national referendum to decide the future of the country's vast natural gas reserves.