The move was not part of the changes announced earlier this month to the central bank's day-to-day foreign exchange management, traders said, and was instead outright selling of dollars from its international reserves.

The central bank offered to sell a minimum $1 million in the spot market at a minimum rate of 4.1250 reais per dollar. The exact amount sold is unknown, but should be apparent in the central bank's weekly FX flows data next week, traders said.

The announcement caused a sharp rebound in the real to around 4.1340 per dollar from an 11-month low of 4.1940 per dollar, which was within sight of its record low of around 4.25 in September 2015 when Brazil was in one of its worst recessions.

The action, which was separate from the dollar sales announced earlier this month, caught the market off guard.

"It was a surprise. He had just said this morning that the real's move was not atypical compared to other emerging currencies," said a fund manager in Sao Paulo, referring to remarks made by central bank president Roberto Campos Neto.

"Also, this was done at lunchtime when liquidity is low. I think the message from central bank is: 'we won't be predictable'," the fund manager said.

Earlier on Tuesday the central bank sold $550 million in the spot market, but that was part of the program announced earlier in the month for the sale of $3.85 billion in the spot market and sale of the same amount in reverse currency swaps.

Some traders attributed part of the real's renewed weakness on Tuesday to Campos Neto's testimony to the Senate's economic affairs committee that the real's recent moves were "well within" normal ranges.

The real has lost about 8% of its value against the dollar in August, under pressure from growing worries surrounding global trade and economic growth, and expectations that Brazil's central bank will continue to cut interest rates.

The real is on course to be one of the world's worst-performing currencies this month, behind the Argentine peso.