(Adds results of second poll)
By Anthony Boadle
BRASILIA, Sept 29 (Reuters) - President Dilma Rousseff's expected victory margin over closest rival Marina Silva has surged to 9 percentage points in a second-round runoff in Brazil's presidential election, an opinion poll showed on Monday, causing stocks and the real currency to tumble.
Rousseff would win the runoff with 47.7 percent of the votes against 38.7 percent for Silva, polling firm MDA said, widening her lead from the one-point advantage she had in the previous survey by the firm last week.
Another survey released later on Monday by the Vox Populi polling firm showed Rousseff with a seven-point lead over Silva in a runoff, unchanged from a week earlier.
Polls by MDA and Vox Populi are not as closely followed as surveys by the bigger research firms Datafolha and Ibope, which use larger samples of voters and conduct polls more frequently.
The first round of Brazil's presidential election is on Sunday. No candidate is expected to win more than 50 percent of the valid votes, which would trigger a runoff round between the top two vote-getters on Oct. 26.
The election is being closely watched by many investors who would like to see Silva, a popular environmentalist who has embraced pro-market policies, unseat leftist Rousseff and end 12 years of Workers' Party rule.
Some investors blame Rousseff's policies for the stagnation of Latin America's largest economy, and markets were weighed down by a Datafolha poll on Friday that was confirmed by Monday polls showing the incumbent consolidating her re-election chances.
The Brazilian real retreated to its weakest level in nearly six years on Monday and the benchmark Bovespa stock index fell 4.52 percent, its biggest one-day drop in over three years.
In a first-round vote, Rousseff would take 40.4 percent of the votes and environmentalist Silva 25.2 percent, the MDA poll showed. That compares with 36 percent for Rousseff and 27 percent for Silva in the previous MDA poll. Support for centrist candidate Aecio Neves, the market favorite stuck in third place, has risen to 19.8 percent from 17.6 percent last week.
Silva, who was thrust into the race when her party's original candidate was killed in a plane crash last month, initially surged on the support of voters who back her pledge to clean up Brazilian politics. But criticism of her ability to govern without Brazil's traditional parties from the Rousseff and Neves campaigns has eroded her support.
The MDA poll showed that Silva's rejection rate, or those who say they would never vote for her, has risen to 42.5 percent from 29.3 percent in late August, while Rousseff's rejection numbers have fallen four points to 41 percent.
Silva could still defeat Rousseff in a second-round runoff when she would have the benefit of equal media exposure required by Brazilian electoral law. She is also expected to win the votes of the bulk of supporters of the other opposition candidate Neves.
Analysts expect Rousseff's advantage over Silva to continue to increase this week but at a slower pace. Still, they are reluctant to bet on a winner because the race could change in a runoff.
"At the current stage of the campaign, we think an advantage of such magnitude in runoff simulations would not be sufficient for one to categorically affirm that the incumbent is indeed the favorite," Credit Suisse analysts said in a note to clients.
The MDA poll, which was commissioned by the transport industry lobby CNT, surveyed 2,002 people on Sept. 27-28. It has a margin of error of plus or minus 2.2 percentage points. (Editing by Meredith Mazzilli, Marguerita Choy and Ken Wills)