By Ana Mano and Tatiana Bautzer

SAO PAULO (Reuters) - A minority investor in Oi SA <OIBR4.SA>, Brazil's largest fixed-line phone carrier, has called for the replacement of most of its board after the company filed for the country's biggest-ever bankruptcy protection.

Nelson Tanure, a Brazilian investor with a contentious track record, and partners have been buying up shares through a fund controlled by Bridge Administradora de Recursos Ltda, according to four sources familiar with the matter.

In a late Thursday filing, Oi said Bridge, acting on behalf of a fund holding 6.6 percent of Oi's capital, had given eight days to call a shareholder meeting to replace board members. The company said it was reviewing the request.

The shareholder activism underscores deep divisions on Oi's board that derailed recent negotiations with creditors. Battle lines on the board remain from an ill-fated 2013 merger with Portugal Telecom, pitched as a lifeline for Brazil's struggling national champion, which soured when poor cash management by the Portuguese partner came to light.

A Bridge representative declined to identify investors in the fund or to answer other questions about plans for Oi. Tanure declined through representatives to comment on the matter.

A source close to Tanure said he was the main investor and chief representative of the fund, adding that he went to Ontario and New York recently in a bid to organize an investor group.

Oi said in its filing that the activist investor aims to replace five board members appointed by Pharol SGPS <PHRA.LS>, formerly Portugal Telecom, and a former chief financial officer.

Those Pharol board members balked at former Chief Executive Bayard Gontijo's negotiations last month aimed at a debt-for-equity swap that would have diluted existing shareholders, sources said at the time. The impasse triggered Gontijo's resignation, followed by the filing for bankruptcy protection.

Oi's common shares <OIBR3.SA>, which have doubled in value since the company's bankruptcy filing nearly three weeks ago, were up 4.8 percent in Friday trading.

Pharol is currently the largest shareholder in Oi, with 27 percent of voting shares, a steep barrier for any rival investor group looking to change the board at a shareholder assembly.

Tanure last made news in Brazil's telecommunications industry with a lawsuit against the controlling shareholder of Oi's rival, TIM Participações SA <TIMP3.SA>, in 2012.

Through another investment vehicle, Tanure accused Telecom Italia SpA <TLIT.MI> of abusing its control of TIM by appointing a chief executive the company knew was a target of an Italian investigation into irregular SIM card activations.

Tanure, who made a fortune buying troubled shipyards and a small bank in the 1990s, entered the telecom sector through long-distance operator Intelig Telecom, which was acquired by TIM in 2009.

(Additional reporting by Silvio Cascione and Alberto Alerigi Jr.; Writing and additional reporting by Brad Haynes; Editing by Bernadette Baum and Andrew Hay)