The newly appointed head of France's secret service, Gen. Rene Imbot, clenched his fists last night as he discussed the "Greenpeace affair" that has damaged French prestige around the world and shaken the country's Socialist government.

"I have cut off the rotten branches," announced the French Foreign Legion veteran, looking grim and pugnacious as he glared into the television cameras. "From now on, any information that someone claims to collect from this service will be a lie, because I have locked up this service."

A few hours before Gen. Imbot made his surprise television appearance from outside the defense minister's office, a laconic communique was issued by the Elysee presidential palace. There would be no comment on the latest newspaper allegations of a high-level cover-up of French involvement in the sinking of a Greenpeace ship in New Zealand July 10. All the necessary explanations had already been provided. The ship, the Rainbow Warrior, was scheduled to lead a protest flotilla to the area where France does its nuclear testing.

The message was clear. After two months of a steadily spreading political scandal described by some commentators as "Watergate on the River Seine," France's civilian and military leaders have now decided to retreat no further. The sacrifice of the defense minister, Charles Hernu, and the head of the General Directorate for External Security (DGSE), Adm. Pierre Lacoste, has provided them with a common line of defense.

Whether a turning point has been reached in the tortuous matter will depend on a variety of factors ranging from the willingness of the French press and opposition to pursue the scandal further to the approach of crucial legislative elections in March 1986. Also important is the legal process in New Zealand, where two French agents are awaiting trial on charges of murdering a Greenpeace photographer killed when the Rainbow Warrior was sabotaged.

The French government last week acknowledged for the first time that agents of the security directorate, acting under orders, had blown up the Greenpeace ship to prevent it from leading the campaign against French nuclear tests in the South Pacific. In a television broadcast Wednesday, Prime Minister Laurent Fabius pinned responsibility for both the original decision and the subsequent cover-up on Hernu and Lacoste.

"A political head had to fall. If Hernu had been sacrificed right at the beginning, that would have been sufficient to put a stop to the scandal. The fact that it has taken this long, after so many revelations in the press, has left the authorities still vulnerable," commented Michel Crozier, a French "Americanologist" who made a close study of Watergate.

Fresh impetus for the Greenpeace affair could be provided by revelations about the still mysterious role of some of President Francois Mitterrand's closest advisers in approving the original operation. Newspapers and right-wing opposition politicians have also challenged a claim by Fabius that he did not find out the truth about the Greenpeace operation until after Hernu's resignation.

"This affair will not be over as long as doubts remain about the truth, and the truth is something that the government has yet to reveal to us," remarked Jean-Claude Gaudin, parliamentary floor leader of the center-right Union for a French Democracy party.

Parallels with Watergate can, however, be misleading. There is a general reluctance in France on the part of both opposition politicians and the media to challenge the head of state directly.

Public opinion polls show that, even though a majority of French citizens believe that Mitterrand knew about the Greenpeace affair right from the beginning, only 20 percent think he should resign because of it. Although 78 percent of those questioned described the sinking of the Greenpeace ship as "unacceptable," only 21 percent thought the scandal would have a significant impact on the next elections.

Le Monde, the newspaper that delivered the death blow to Hernu by revealing a cover-up in the Defense Ministry about the sabotage of the Rainbow Warrior, has refrained from linking either Mitterrand or Fabius to the scandal. The three principal leaders of the opposition -- former president Valery Giscard d'Estaing and former prime ministers Jacques Chirac and Raymond Barre -- have yet to make any substantive comment.

French commentators depicted this week's developments as part of a tacit agreement between the Socialists and the military, who have traditionally been mistrustful of each other. Political responsibility for the fiasco has been assumed by the government, which has gone out of its way to maintain the prestige and honor of the Army.

In return, according to political sources, senior military leaders are helping the government put a stop to further damaging revelations. Leakers within the security directorate have been muzzled. A reported threat by Adm. Lacoste -- that he would provide details about the "civilian command" of the operation if the secret services were turned into a scapegoat -- now seems unlikely to be realized.

Apart from Lacoste, the only other figure in a position authoritatively to challenge the government's version of events is former defense minister Hernu. But he, too, appears to have agreed to keep quiet.