(Updates prices, adds analyst quote, Araujo and Mantega comments)
By Asher Levine
SAO PAULO, Sept 29 (Reuters) - The Brazilian real touched its weakest level in nearly six years on Monday and the benchmark Bovespa stock index was on track for its biggest one-day drop in more than a year after a poll showed President Dilma Rousseff gaining on challenger Marina Silva ahead of Sunday's election.
The poll, released late Friday by research firm Datafolha, showed Rousseff advancing in first-round voter intentions. In a potential second-round runoff, where Silva had once led by as much as 10 points, Rousseff held 47 percent against Silva's 43 percent.
Brazilian investors have been heavily critical of Rousseff's government for implementing policies that have gone against minority shareholder interests in state-run companies and a tendency to enact one-off stimulus measures rather than structural reforms.
The real plunged as much as 2.54 percent in early trade to 2.478 per dollar, its weakest level since late 2008, before paring losses to about 1.25 percent.
Many traders see Brazil's risk premium declining under a different administration, which would help attract foreign investment and support the real.
"If (Rousseff's advance) is reconfirmed in the next polls, the sky is the limit for the dollar," said Mario Battistel, head of currency trading at Sao Paulo brokerage Fair, adding that the real could pass 2.50 per dollar in the short term.
In the past the central bank has stepped up its intervention in the currency market when the real weakens sharply in an effort to limit pass-through to inflation.
"It is quite possible that the (central bank) has shifted its 'comfort zone' to adjust to a broad dollar-strengthening trend," wrote Brown Brothers Harriman's Ilan Solot in a client note. "So maybe 2.50 is the new top. We will only know this in time."
Central bank director Carlos Hamilton de Araujo said on Monday that the bank is paying attention to the effects of a weaker currency on inflation, but he stressed that the pass-through, or the increase in import prices as a result of a weaker currency, is lower now than in previous years. Araujo also told reporters that the bank's daily intervention program has worked well, but he declined to elaborate.
The Bovespa index dropped below 55,000 points for the first time since mid-July as shares of widely traded state-run firms sank. Oil producer Petroleo Brasileiro SA, known as Petrobras, fell about 10 percent, and was on track to close with its biggest one-day loss since Nov. 2008.
Investors in Petrobras have lambasted the Rousseff government for forcing the company to import fuel at global prices and sell it at a loss in the domestic market as a way to tamp down inflation.
Shares of state-run lender Banco do Brasil SA fell about 6.5 percent, on track for their biggest one-day decline since late May. The bank also saw its shares sink due to government intervention in 2012 after the Rousseff government forced it to lower lending spreads in an effort to cheapen credit and boost consumption.
Finance Minister Guido Mantega shrugged off the market moves on Monday, telling reporters that "some players like to use the elections to move the market". (Additional reporting by Walter Brandimarte and Bruno Federowski; Editing by Meredith Mazzilli; and Peter Galloway)