By Paul Kiernan 
 

RIO DE JANEIRO--Brazilian stocks snapped a three-session winning streak and closed lower Friday as investors awaited results of a new public-opinion poll on the country's October presidential elections.

The market's benchmark Ibovespa index of most-traded stocks fell 0.2%, or 118.02 points, to close at 60681.98. Trading volume was heavy at 9.09 billion Brazilian reais ($4.05 billion).

Brazil's real exited active trading Friday at BRL2.2424 to the dollar, according to Tullett Prebon via FactSet, slightly stronger its Thursday close of RL2.2444.

The stock market was weighed down by heavyweight mining company Vale SA, shares of which tumbled 1.8% to BRL25.00. The firm has seen its market value drop in recent days alongside prices for iron ore, its main cash generator, which fell this week to a five-year low on concerns about oversupply and weak demand from China.

Investors are also keeping a close eye on opinion polls. The Ibovespa has risen by more than one-third since mid-March on hopes that left-leaning President Dilma Rousseff will lose her bid for re-election. Her chances have taken a hit in the past two weeks by the sudden rise of opposition candidate Marina Silva, who is now seen defeating Ms. Rousseff in a likely runoff.

Two polls this week, by Ibope and Datafolha, showed Ms. Silva with a slightly smaller lead than before. A third is due out Saturday.

"The market is going on standby while the electoral panorama shapes up," said Bruno Goncalves, an analyst at Alpes brokerage in Sao Paulo. "Political change could bring about some excitement, and improve consumer and business confidence. But no one is denying that next year will be a bad year for the economy, no matter who wins."

Shares of state-run oil company Petrobras edged 0.1% higher to close at BRL22.82, while telecommunications firm Oi SA was the biggest gainer, rising 4.6% to BRL1.60.

Shares in meatpacking giant JBS fell 3.7% to BRL9.77 and state-owned Banco do Brasil fell 1.6% to BRL34.20.

Write to Paul Kiernan at paul.kiernan@wsj.com

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