By Ricardo Brito and Rodrigo Viga Gaier
BRASILIA (Reuters) - Brazil's electoral authorities are preparing for a competitive election on Sunday with a result that may be contested by far-right President Jair Bolsonaro if he loses to his leftist adversary, who has a narrow lead in recent opinion polls.
Brazil's Superior Electoral Court (TSE), led by justices from the Supreme Court, is ready for Bolsonaro to dispute any victory by former President Luiz Inacio Lula da Silva, potentially mobilizing angry demonstrations, according to people familiar with the court's preparations who requested anonymity to share the information.
"What we see is a series of things intended to destabilize and disrupt the electoral process," a senior judicial source told Reuters, while playing down the risk of widespread unrest.
"We have to follow what will happen closely and keep in touch with the security forces to avoid riots in the streets," said the source.
Four sources told Reuters the TSE and Supreme Court have plans to reinforce security for judges and institutions on Sunday, and they expect tensions could last until the result of the presidential election is certified on Dec. 19.
Two polls on Thursday showed Lula maintaining a lead of 5 or 6 percentage points, in line with his advantage in the first round of voting on Oct. 2.
Financial markets have largely priced in a Lula victory, but the uncertainty about a contested result has some on edge.
"There is apprehension about whether the election result will be respected," said J.P. Morgan equity strategist Emy Shayo. "What everyone wants right now is a definition, regardless of what it is," she said.
Bolsonaro, a far-right populist who has argued without evidence that Brazil's electronic voting system is vulnerable to fraud, stepped up his attacks on the TSE this week. He accused the court of mishandling a complaint that radio stations had given Lula more air time for campaign ads.
The TSE rejected the complaint in a Wednesday ruling due to lack of evidence and asked Brazil's top prosecutor to investigate the Bolsonaro campaign for possible intent to disrupt the election.
One of the president's sons, Senator Flavio Bolsonaro, said on social media that his father was a victim of the "biggest electoral fraud ever seen." Another son, Congressman Eduardo Bolsonaro, told journalists that to address the issue properly might require postponing the election.
Senior Bolsonaro campaign officials have said publicly that a postponement of the election is not up for discussion. The issue split the president's advisers, some of whom consider the strategy a distraction from winning over swing voters, according to campaign sources.
Another senior judiciary official told Reuters that Bolsonaro's pivot toward attacking the TSE itself had made the post-election outlook "unpredictable."
Bolsonaro's allies have called for supporters turn out in Brasilia's central esplanade on Sunday to follow the vote count in what organizers called a "Victory Party."
Bolsonaro has also asked supporters to stick around voting stations until they close at 5 p.m. (2000 GMT) on Sunday, which critics say could intimidate voters and lead to clashes.
Lula, a former labor union leader who founded the Workers Party and led Brazil from 2003-2010, has called on voters to defend Brazil's democracy from Bolsonaro's "neofascism."
Adding to the climate of uncertainty, Bolsonaro, a former army captain has pushed the military to publicly endorse his allegations that the voting system is vulnerable to fraud. The armed forces checked some voting machines during the first-round vote to be sure paper receipts lined up with the results transmitted digitally, but they did not report their findings.
Lula aides, including former Defense Minister Celso Amorim, have said they are ready for unrest but they see no way the armed forces would back any unconstitutional moves by Bolsonaro.
(Reporting by Ricardo Brito in Brasilia, Rodrigo Viga Gaier in Rio de Janeiro; Additional reporting by Paula Arend Laier in Sao Paulo; Writing and additional reporting by Anthony Boadle in Brasilia; Editing by Brad Haynes)