BBC Brazilian security forces and protesters have clashed in Barra da Tijuca, near Rio de Janeiro, where the Brazilian government is auctioning off exploration rights for a huge oilfield on Monday.
Members of the National Security Force fired tear gas and rubber bullets to try to disperse some 300 protesters.
Several protesters have been injured.
President Dilma Rousseff had ordered tight security after violent demonstrations in Rio last week.
Local media said a small number of protesters tried to set a car alight while others tried to block cars carrying officials from Brazil’s Mining and Energy Ministry from getting to the hotel where the auction is taking place.
“There were bizarre scenes. Riot police firing tear gas and stun grenades, not just against protesters, but also on to the beach, with hundreds of tourists and sun worshippers looking on incredulously,” the BBC’s Wyre Davies reported from Rio de Janeiro.
Among the protesters are members of various unions representing oil workers, who have been on strike since Thursday at more than 40 oil platforms and refineries.
The unions accuse the government of “selling off” Brazil’s riches.
Eleven international oil companies are said to be interested in the Libra oilfield, and nine of them have so far reportedly paid the required registration fee.
The Libra offshore oilfield is located at a depth of some 5,000m (16,500ft), below a thick layer of salt sediments, which makes its exploration extremely challenging.
The Brazilian government hopes that by bringing foreign expertise on board it will be able to exploit Libra’s estimated 8-12 billion barrels of recoverable oil reserves.
“We’re taking over this immense wealth that lies under the sea and on land. It is not available to us if it stays where it is,” Mines and Energy Minister Edison Lobao told AFP news agency.
But critics say Brazil should protect the monopoly of exploration of its oil reserves.
Others argue that the rules set by the government for the auction have scared off big players such as BP and Exxon for allegedly allowing too much state intervention.
The Libra field belongs to the Brazilian government’s National Petroleum Agency (ANP) and is part of huge oil reserves discovered in 2010 under a layer of rock, sand and salt beneath the seabed.
The discovery potentially doubles Brazil’s known oil reserves.