Brazil's finance minister defended the planned Oct. 21 auction of the nation's largest oil field amid criticism from unions, saying it would pave the way for a huge influx of investment in the economy.

It will be very important as the "first step to attracting a volume of investment greater than $180 billion" in the coming years, Guido Mantega said Thursday of the upcoming auction in the largest pre-salt field, Libra.

The pre-salt frontier, a series of ultra-deep oil fields that were discovered in recent years and stretch for some 800 kilometers (500 miles) off the coasts of the southeastern states of Espirito Santo, Rio de Janeiro, Sao Paulo and Santa Catarina, could dramatically increase Brazil's proven-reserve tally and transform the country into a major crude exporter.

But accessing those fields, so-named because they are located under water, rocks and a shifting layer of salt at depths of up to 7,000 meters (22,950 feet) below the surface of the Atlantic, will be very costly and pose an enormous technical challenge.

The awarding of pre-salt oil field and infrastructure contracts to private companies will mobilize the economy and spur growth in 2014 and in subsequent years, Mantega told a press conference.

But several grassroots movements and unions launched protests Thursday against what they call the privatization of the Libra field, which may hold between 8 billion and 12 billion barrels of oil equivalent.

In Brasilia, hundreds of people occupied the headquarters of the Mines and Energy Ministry to demand the suspension of the auction.

The first pre-salt auction to be held under a new regulatory regime that requires state-controlled Petrobras be the sole operator and have a minimum 30 percent stake in all those fields, it also is opposed by oil workers, who went on strike Thursday, and fishermen's unions in Rio de Janeiro.

Nine companies have registered for the auction, including the China National Petroleum Corporation, Royal Dutch Shell, Colombia's Ecopetrol, France's Total, China National Offshore Oil Corporation and Spanish-Chinese consortium Repsol-Sinopec.

Brazilian state oil company Petrobras also plans to bid, although energy super majors ExxonMobil, Chevron and BP opted not to participate.

Under the new regulatory framework, Petrobras' partners in the pre-salt region would not own any of the reserves but could share in the profits from oil and natural gas sales. EFE