RIO DE JANEIRO — Brazil's government was set to announce the result of an auction of its massive Libra offshore oil block Monday under new rules giving state-run petroleum company Petrobras more control over reserves.

Hundreds of protesters wanting to halt the auction clashed with police outside the luxury hotel where the auction results were to be announced, with security officials firing tear gas and rubber bullets at demonstrators.

Oil industry analysts decried the new production-sharing rules for auctioning offshore reserves discovered in recent years, which could hold upward of 100 billion barrels of oil and make Brazil one of the globe's largest exporters. Libra itself could hold up to 12 billion barrels.

The new rules mandate that Brazil's state-run oil company Petrobras be the sole operator of the fields and hold a minimum 30 percent stake in them. Many say that will reduce the profit for private oil companies and delay the development of Brazil's fields by making them less attractive for outside investors.

Adriano Pires, one of Brazil's top energy analysts, called the new rules "very interventionist" and said the auction was already a big disappointment, with just 11 companies taking part, while the government had hoped to attract 40. He chalked that up to the new rules for ultra-deep oil that will be expensive to produce.

"We are talking about non-conventional oil located 6,000 to 7,000 meters deep," he said. "The $500 billion that will have to be invested over the next 12 to 15 years made companies conclude that the rate of return made participating in the auction unattractive."

Brazilian officials are counting on the oil riches to quickly catapult the nation to developed-world status. State officials are locked in arguments about how to split royalties that haven't surfaced, the Navy is buying submarines to protect the fields and left-wing protesters like those demonstrating Monday demand that the entire industry be nationalized so Brazil doesn't share a drop of the wealth.

But the technology hurdles to reaching the riches are intensely challenging, even for Petrobras, considered a world leader in offshore development.

The deep-water reservoirs lie some 185 miles (300 kilometers) offshore in the Atlantic, more than a mile (kilometer) below the ocean's surface and under another 2.5 miles (4 kilometers) of earth and corrosive salt. The salt beds can break loose and shear off piping, making it one of the toughest substances to drill.

With a slowing economy and delays in producing that offshore oil, some say the Brazilian government will loosen rules to make them more business friendly when the next auction is held in two to three years.

The New York-based Eurasia Group said in a research note that Petrobras' "growing operational and financial constraints" along with government pressure to stoke a lagging national economy means changes are expected at the next auction.

"Allowing international oil companies to develop the pre-salt side by side with Petrobras would kill two birds with one stone," Eurasia Group wrote. "It would lead to a quicker pace of production in the pre-salt with more investments, and provide needed relief to Petrobras."

It added that "it isn't lost on government officials that the shale gas and tight oil technological revolution in North America has reduced Brazil's leverage to attract capital."

About 300 protesters hoping to halt the oil auction tossed tear gas canisters back at police and overturned the car of one local TV channel, setting it aflame. Among the demonstrators were the masked, black-clad "Black Bloc" anarchists who have a growing role in Brazil's steady drumbeat of protests.

The demonstration was originally called by striking oil workers, whose union has long been against any foreign involvement in Brazil's petroleum production.

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Associated Press writer Stan Lehman in Sao Paulo contributed to this report.