* Cenbank conducting survey go gauge demand for dollars
* Real gains in futures market after announcement
RIO DE JANEIRO, Dec 7 The Brazilian currency was set to strengthen early next week after the central bank warned late on Friday it was conducting a survey to gauge demand for dollars in the foreign exchange market.
The real ended 0.6 percent weaker at 2.0904 per dollar on Friday, interrupting a string of four winning sessions that had been spurred by a series of government measures designed to support the currency.
Right after the market closed, however, a central bank spokesman said the bank was in touch with currency desks at key banks in Brazil to gauge demand for traditional currency swaps -- derivatives that emulate the sale of dollars in the future market -- and for dollar sales with repurchase agreement.
Those were exactly the tools the central bank used on Monday when it acted to halt a slide that had taken the real to its weakest level in more than 3-1/2 years.
Although Brazil's spot currency market was closed when the central bank started its survey, dollar futures contracts moved sharply at the BM&FBovespa.
The contract for January 2013, the most liquid at the exchange, showed the real strengthening to 2.082 per dollar from 2.098 reais per dollar minutes before the announcement.
The central bank will announce its swap or dollar auctions through its regular messaging system if it finds demand for them, the bank's spokesman said.