Several protesters were injured after members of the National Security Force fired tear gas in an attempt to disperse some 300 protesters.
President Dilma Rousseff had ordered tight security after violent demonstrations in Rio last week.
A small number of protesters 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.
Among the protesters are members of 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 are thought to have paid the required registration fee.
The Brazilian government hopes that by bringing foreign expertise on board it will be able to exploit Libra's estimated eight to 12 billion barrels of recoverable oil reserves.
Energy Minister Edison Lobao said: "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."