* Macron emphasised desire for alliance stability -Elysee official
* Abe said alliance's future up to private shareholders -spokesman
* Japan's Seko sent protest letter to Le Maire -newspaper
* Ghosn's detention extended again -Japanese media (Adds quotes from Abe spokesman)
By Kaori Kaneko and Michel Rose
TOKYO/PARIS, Nov 30 (Reuters) - France and Japan's leaders met for bilateral talks to avert a diplomatic row over the Renault-Nissan-Mitsubishi alliance on Friday following the surprise arrest of its Chairman Carlos Ghosn in Japan.
With the carmaking alliance facing its biggest test after the ousting of Ghosn as chairman of Nissan and affiliate Mitsubishi over financial misconduct allegations, President Emmanuel Macron sat down with Prime Minister Shinzo Abe at the G20 summit in Buenos Aires.
Ghosn's arrest to face accusations including the under-reporting of income has triggered new attempts by Nissan to weaken Renault's control of the Franco-Japanese alliance, adding to challenges facing Macron at home.
Macron, whose government has repeatedly pressed Japan to share evidence unearthed by Nissan's internal investigation into Ghosn, "restated his firm wish that the alliance should be preserved, along with the stability of the group", an Elysee official said after Friday's meeting with Abe.
Abe said it was important to "maintain a stable relationship", according to a spokesman for the Japanese leader.
"However, he said the future of the alliance is up to the private-sector shareholders. The government of Japan does not prejudge the future of the alliance," the spokesman said.
The French official quoted Abe as telling Macron that "the legal process must be allowed to take its course."
Tokyo authorities on Friday extended Ghosn's detention for a second time, by the maximum-allowed 10 days, local media reported. Prosecutors must file charges by Dec. 10 or arrest Ghosn for new crimes to hold him beyond that date.
Tokyo prosecutors declined to comment. Nissan did not immediately respond to a request for comment.
Ghosn's detention has left the global auto alliance without its leader and main interlocutor with the French government, which owns 15 percent of Renault and wants to maintain the ownership structure enshrining its control of the partnership.
But Nissan Chief Executive Hiroto Saikawa has made clear that Nissan wants to weaken the control of its smaller parent as it carries out a governance review.
Renault's 43.4 percent Nissan stake ensures an effective voting majority at shareholder meetings, while Nissan's reciprocal 15 percent Renault holding carries no voting rights.
The Macron-Abe talks came as a diplomatic spat was brewing over comments by French Finance Minister Bruno Le Maire, who said that his Japanese counterpart Hiroshige Seko had agreed in earlier talks that the cross-shareholdings should be left unchanged.
Seko denied any such agreement through a rare official letter of protest sent to Le Maire, Japanese daily newspaper Mainichi Shimbun reported.
Officials at both of the ministries declined to comment on the letter or the incident.
Le Maire also came in for criticism from Renault staff representatives concerned for the alliance.
"The government should know their place and stay there," said a union official at the French carmaker. "This kind of overreaching may be counter-productive."
As economy minister, Macron masterminded the French government's surprise increase to its Renault stake in 2015, raising concern within Nissan that the Elysee sought to wield more influence over the Japanese company.
Renault, Nissan and Mitsubishi "emphatically reiterated" their commitment to the alliance on Thursday after executives met in Amsterdam for the first time since Ghosn's arrest.
A review of the capital structure was not discussed at the meeting, Mitsubishi CEO Osamu Masuko said. (Reporting by Kaori Kaneko, Chris Gallagher and Linda Sieg and Elaine Lies in Tokyo, Michel Rose and Laurence Frost in Paris, and by Daniel Flynn and Maximilian Heath in Buenos Aires; Writing by Laurence Frost and Chang-Ran Kim; Editing by Keith Weir, David Goodman and Sonya Hepinstall)