SAO PAULO, May 27 (Reuters) - Brazilian state power company Eletrobras launched on Friday a bid to sell up to 30.69 billion reais ($6.45 billion) of mainly new shares, in what may be President Jair Bolsonaro's last chance for a major privatization before October elections.

Eletrobras said it had filed a share offering request with the country's securities and exchange commission, with the estimated value based on Thursday's closing price of 44 reais per common share.

Privatizing Latin America's largest utility is seen as crucial for Bolsonaro's government as he has so far delivered few of the state asset sales he pledged before taking office in 2019.

It would come just months before Bolsonaro, a self-proclaimed free markets advocate, is set to take on election front runner, former President Luiz Inacio Lula da Silva - an avowed opponent of privatizations.

Unlike with some other big state asset sales, no single investor, foreign or domestic, will be able to take control of the company through the process, which will set a 10% ceiling on individual stakes. That leaves some uncertainty over the future strategic direction of the company, whose largest single shareholder will remain the government.

The new shares will be created through a capital increase diluting the government and other existing shareholders, and would leave the new block of investors controlling about a third of the company.

The government currently holds 72% of Eletrobras' common shares, including its own stake and those held by state development bank BNDES and the national development fund (FND), according to Refinitiv data, a stake that would fall to 45% after the privatization.

Newspaper Valor Economico reported on Friday that Singapore's sovereign wealth fund GIC, Brazilian asset managers SPX, Truxt, Squadra and Itau Asset, U.S.-based Zimmer and GQG Partners, and British firm RWC would take part in the transaction as anchor investors.

A series of doubts remain as the government rushes to privatize Eletrobras and secure enough demand for the shares amid rising interest rates, said Gustavo Pazos, an analyst at brokerage Warren, adding the company's future governance remained a question mark given its likely diffuse ownership.

"The market expects Eletrobras to cut administrative and operating expenses, but it is really hard to say by how much," said Pazos.

Centrais Eletricas Brasileiras SA, as the company is formally known, said the transaction would comprise a primary offering of 627,675,340 new shares and a secondary offering of 69,801,516 shares currently held by BNDES.

The offering in Brazil may also be increased by 15% through a green shoe option, which is aimed at price stabilization, it added. Including the green shoe option, the transaction would reach 35.18 billion reais. American Depositary Shares will be sold in the United States.

Preferred shares in Eletrobras were up 0.5% at 42.72 reais in midday trading in Sao Paulo, outperforming Brazil's Bovespa stock index, which was near flat.

($1 = 4.7583 reais) (Reporting by Gabriel Araujo and Tatiana Bautzer Editing by Mark Porter and Mark Potter)