President Francois Mitterrand today invited leaders of countries critical of French nuclear tests in the South Pacific to visit the remote atoll where the experiments are conducted.

Mitterrand, just back from a rapid visit to the nuclear testing center on Mururoa, announced measures designed to strengthen the French presence in the South Pacific. He said France would continue to carry out the tests as long as necessary, despite the opposition of most of the countries in the region.

The visit to Mururoa, a sliver of land on the other side of the globe, has coincided with controversy over the sabotage of an environmental protest ship belonging to Greenpeace. Two French secret service agents are awaiting trial in New Zealand on charges of conspiring to blow up the ship and killing a Portuguese-born photographer in the explosion on July 10.

In a brief television declaration, Mitterrand said he was inviting leaders of South Pacific nations to Mururoa so that they could witness the absence of any danger of radioactive contamination. He asked for facilities to be extended to French scientists to study the effects of radiation of British nuclear tests in the Australian desert, made some years ago.

Mitterrand also announced the creation of a center for advanced French studies in the region. Later, at an informal meeting with journalists, he explained that the 300,000 French speakers in the region were vastly outnumbered by about 20 million English speakers.

France's principal territories in the South Pacific are Tahiti, Mururoa and New Caledonia. Separatist agitation on New Caledonia, where election of an island assembly is due next month, already has posed a political problem for Mitterrand's Socialist government.

New Zealand Prime Minister David Lange told a television interviewer: "That's all this trip has been about . . . a hit-and-run mission to say that France is a Pacific power, France is a nuclear power, and to make an obscene gesture at everyone else who thinks otherwise."

Mitterrand brushed aside Lange's remarks as "unconsidered." The state of relations between France and New Zealand has been reflected in a decision to postpone a planned visit to Paris by New Zealand's deputy prime minister, Geoffrey Palmer.

Mitterrand appeared to pull back from a remark attributed to him by his press spokesman on Mururoa in which he described opponents of French nuclear tests as "adversaries" of France.

"France does not have enemies in the South Pacific. It simply intends to see that its rights are respected. Let no one be astonished about that," said Mitterrand, speaking from a lectern shaped in the form of the prow of a ship and decorated with the French national colors.

The president later hinted that he may meet the New Zealand prime minister eventually in an effort to overcome the diplomatic crisis that has blown up between the two countries. He described a message from Lange suggesting such a meeting as "an excellent initiative."

While on Mururoa, Mitterrand announced that work would begin soon on a strategic military base in New Caledonia. The decision was made earlier this year when he visited the French territory following riots in which more than 20 Kanak separatists and European settlers were killed.