SAO PAULO, Aug 17 (Reuters) - Brazilian President Luiz Inacio Lula da Silva holds a tight lead over Senator Flavio Bolsonaro, his main right-wing challenger, in a potential runoff ahead of October's election, a BTG Pactual/Nexus poll showed on Monday.
In a simulated second-round runoff, the leftist Lula would receive 47% of voter support compared with 44% for Bolsonaro, unchanged from last week's poll.
The poll has a margin of error of two percentage points.
In a first-round scenario, Lula would lead with 41% of the votes, followed by Bolsonaro at 36%, each up one percentage point from a week earlier.
Senator Bolsonaro is the eldest son of former President Jair Bolsonaro.
If no candidate wins more than 50% of valid votes in the first round, the two frontrunners advance to a runoff.
The poll surveyed 2,003 people from August 14 to 16.
(Reporting by Fernando Cardoso; Editing by Hugh Lawson)