The Brazilian government has begun deploying troops on the streets of Rio de Janeiro ahead of a major auction for oil exploration rights.
Some 1,100 soldiers sealed off the seafront hotel, where the auction of the huge offshore Libra field is due to take place on Monday.
President Dilma Rousseff ordered tighter security after violent demonstrations in Rio last week.
Anti-government protesters have also threatened to march on the auction.
In scenes similar to country-wide protests in June, petrol and gas bombs were thrown last Tuesday after marches in support of striking teachers.
The forthcoming auction of the Libra deepwater oilfield, some 230km (140 miles) off the coast of Rio de Janeiro, added to the tension.
On Thursday, oil workers launched a strike against the sale, disrupting production at more than 40 oil platforms and major refineries.
Eleven 11 international oil companies are said to be interested in the oilfield but it remains controversial in Brazil.
Some critics say Brazil should protect the monopoly of exploration of its oil reserves.
Protesters took to the streets of Rio de Janeiro and other cities
But the government insists it is not privatising the country’s resources.
“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.
The unions say the Libra oilfield auction will damage national interests by allowing foreign companies to share production rights with state-run oil company Petrobras.
On Sunday, the government said it had defeated 18 of 23 court actions lodged to prevent the sale from happening, at the Windsor Barra hotel in the Barra da Tijuca area of Rio.
Other critics say the rules set by the government for the auction have scared off big players such as British Petroleum and Exxon for allegedly allowing too much state intervention.
The BBC’s Wyre Davies: “The figures are almost mindboggling and the opportunities almost endless”
Thousands of people stated their intention to join demonstrations via social media at protests near the Windsor hotel.
But the army says the area will be cordoned off.
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.
The government expects Libra’s production to peak at 1.4m barrels a day in a decade.
The Libra field has estimated recoverable oil reserves of 8-12bn barrels, according to the National Petroleum Agency.