President Francois Mitterrand came back to Paris tonight and sought to defuse criticism of his government's handling of a crisis in New Caledonia by announcing that France will strengthen its military presence on the troubled Pacific island.

Speaking after returning from a surprise 12-hour visit to the territory, the president also announced that he would call the National Assembly into special session to extend a state of emergency declared on the island on Jan. 12. He promised that measures would be taken to prevent a confrontation between the rival communities of native Kanaks and French settlers.

Political analysts here said that Mitterrand's statement appeared to signal a shift of emphasis by the Socialist government, with more importance now being attached to restoring security and less talk about plans for moving New Caledonia toward independence.

Large crowds made up mostly of white residents of Noumea, the island's capital, defied a ban on gatherings yesterday to stage a noisy demonstration against independence while Mitterrand was receiving local leaders. The streets turned red, white, and blue, with French flags displayed in practically every house and shop window.

In his statement tonight, Mitterrand made clear that he still regarded a plan put forward earlier this month by Edgard Pisani, the government delegate in New Caledonia, as a possible basis for a future settlement. Pisani proposed holding a referendum there in July to pave the way for independence in "association" with France by the beginning of 1986. Pisani said France would control the island's foreign affairs.

The president put much greater stress than Pisani, however, on the maintenance of France's presence in the South Pacific and omitted any specific mention of the word "independence."

"France intends to maintain its role and strategic presence in this part of the world. I have asked the prime minister . . . to take all necessary measures to this end, particularly regarding the construction of installations for the strengthening of the military base in Noumea," Mitterrand said.

The announcement that the military outpost on Noumea would be strengthened appeared intended to counter accusations by the right-wing opposition that the government is preparing to "abandon" New Caledonia. Leaders of the neo-Gaullist party even have suggested that the island could become a "Soviet aircraft carrier" in the South Pacific if the French presence is withdrawn.

Similar warnings have been sounded by the U.S. ambassador in Paris, Evan Galbraith. In a radio interview here earlier this month, Galbraith said that New Caledonia ran a risk of becoming a new "Grenada" if revolutionary-backed elements succeeded in gaining control over the island.

A different attitude has been taken by Australia, New Caledonia's largest neighbor, which has urged the speeding up of French decolonization in the region.

About 6,000 French soldiers, paramilitary gendarmes and police -- including 1,000 sent a week ago -- are now stationed in New Caledonia, with the task of ensuring security for a population of 140,000. France also maintains a significant naval and military presence in Polynesia in addition to using the South Pacific as a testing ground for its independent nuclear deterrent.

Mitterrand said measures would be taken to ensure the resumption of economic activity on New Caledonia following an upsurge of violence during the past three months. He said that a nickel mine at Thio, which was closed after the town was occupied last November by militant Kanak separatists, would be reopened.

New Caledonia is believed to have the largest nickel reserves in the noncommunist world.