According to the Datafolha polling firm, challenger Jose Serra is still leading incumbent Mayor Marta Suplicy, though the margin has shrunk to 7 points, 49 percent to 42 percent leading up to Sunday's election.
The firm interviewed 2,205 voters and the poll has a plus or minus 2 percentage points margin of error.
The Ibope firm has Serra leading 49 percent to Suplicy's 40 percent, in a survey of 1,204 people. The poll has a margin of errorof 3 percentage points.
The center-right Serra was leading the leftist Suplicy by 10 points or more just a week ago.
The Sao Paulo race is being closely watched by the country, as it is Brazil's largest city and economic capital. President Luiz Inacio Lula da Silva is keen on seeing Suplicy overcome the odds and beat Jose Serra -- who leads the polls -- considering she is a member of his ruling Worker's Party and a win for her would bolster Lula's re-election bid in 2006.