BRASILIA (Reuters) - Former Brazilian President Luiz Inácio Lula da Silva extended his lead in the run-up to the 2018 presidential election despite a corruption conviction that could bar him from running, a poll showed on Saturday.

The Datafolha survey published in the Folha de S.Paulo newspaper showed Lula winning at least 35 percent support from those polled in a first-round vote, up from about 30 percent in a June poll.

Two potential competitors, former environment minister Marina Silva and right-wing congressman Jair Bolsonaro, trailed with 13 percent to 17 percent of voting intentions.

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Lula would beat all potential contenders in a runoff vote, the poll said, expect in an unlikely contest against Sergio Moro, the judge who convicted him of corruption in July. Moro has repeatedly denied being a candidate.

Millions of Brazilians were lifted from poverty during Lula's 2003-2010 presidency. Many detractors blame him for a political graft that flourished under the Workers' Party, calling his bribery conviction the end to his political career.

Lula may not be eligible to run if his conviction is upheld by a higher court. Datafolha showed 26 percent of voters would vote for a candidate endorsed by Lula in such a scenario.

Sao Paulo state governor Geraldo Alckmin and Sao Paulo city mayor Joao Doria, both from the Brazilian Social Democracy Party (PSDB), had 8 percent of voting intentions each.

Lula was found guilty last month and sentenced to nearly 10 years in prison for accepting 3.7 million reais worth of bribes from engineering firm OAS, the amount of money prosecutors said the company spent refurbishing a beach apartment for Lula in return for his help winning contracts with state oil company Petroleo Brasileiro.

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He also faces other graft charges.

(Writing by Silvio Cascione; Editing by Bill Trott)