President Francois Mitterrand today briefed the French Cabinet on plans to support France's military presence in the South Pacific by visiting the French nuclear testing site of Mururoa Atoll.

The president's surprise trip to Mururoa Atoll on Friday comes as the French Navy is preparing to repulse a flotilla of protest vessels belonging to the environmental organization, Greenpeace, which is heading for the testing zone. Most countries in the region, including Australia and New Zealand, strongly oppose the French tests.

Political analysts here said Mitterrand's decision to fly to the South Pacific was a gesture aimed at recapturing the initiative following an espionage scandal triggered by the sinking of a Greenpeace ship in July. New Zealand police have arrested two members of the French secret services and charged them with blowing up the ship and murdering a Greenpeace photographer who was killed in the explosion.

The Mitterrand trip to Mururoa Atoll could also have domestic political implications during the election campaign for a new National Assembly next March, analysts said. The president stands to benefit by being associated in the public mind with the forceful defense of French interests, an issue on which it is very difficult for the right-wing opposition to criticize him.

The president's gesture was praised today by opposition leaders who are normally quick to seize any issue on which to attack him. The only slightly carping comment came from Gen. Marcel Bigeard, a former Army minister who noted that Mitterrand had opposed the nuclear deterrent during the 1960s.

"Mr. Mitterrand was always against General [Charles] de Gaulle's 'little bomb.' Today, however, he is ready to die for it. I can only approve this affair," commented Bigeard.

The ostensible reason for Mitterrand's trip is to preside over a meeting of a newly created "coordinating committee for the South Pacific" made up of French colonial administrators, military officers and ambassadors in the region.

In a statement in London, Greenpeace chairman David McTaggart described the coordinating committee as looking "disturbingly like a council of war." Mitterrand has ordered the French armed forces to prevent "by force if necessary" any incursion by Greenpeace ships into a security zone of 60 nautical miles around Mururoa Atoll.

Mitterrand will be the first French president since de Gaulle to visit Mururoa, an atoll formed from the crest of a submerged volcano. The atoll, 11,000 miles from France, is populated by about 3,000 French soldiers and civilian nuclear experts.

Since his election as president in May 1981, Mitterrand has developed a taste for dramatic overseas visits. Earlier this year, he flew to the French Pacific island of New Caledonia following an upsurge of violence between native Kanaks and French settlers over independence. In 1983, he visited Beirut after an attack on the French military headquarters there in which 58 French paratroops died.

En route to Mururoa, Mitterrand will stop in French Guyana to observe the launching of the European Space Agency's Ariane rocket.