The new French government, in another gesture of support for revolutionary movements, has authorized the major black guerrilla groups in South Africa and Namibia to open offices here.

The decision, reported today by the Foreign Ministry, is the latest in a series of actions on Third World issues that have etched sharp differences between the Reagan administration and the Socialist government of President Francois Mitterrand.

A Foreign Ministry spokesman said the Southwest Africa People's Organization, or SWAPO, the guerrilla group fighting for independence of Namibia from South African control, and the African National Congress, the leading black rebel group in South Africa itself, have the go-ahead to begin missions here next month.

The spokesman quickly emphasized that the offices would be "informational" and would have "no official status."

"Similar offices already are open in Bonn and London," he added.

The decision nevertheless runs directly counter to the Reagan administration's policy of avoiding confrontation with South Africa over its apartheid policy at home and its continued rule over Namibia, or Southwest Africa, in defiance of U.N. resolutions. It highlighted the new tone in Paris since Mitterrand was elected May 10 and brought to his conduct of foreign relations a long Socialist heritage of support for leftist and revolutionary causes.

Mitterrand reiterated his endorsement of that heritage in an interview to the London Times coinciding with his current visit to Britain. He included his willingness to counter U.S. policy on the same issues. Asked whether, as president, he would continue to back leftist revolutionary groups, he replied: "Yes, authentically revolutionary movements. The commitment of friendship with the United States does not include supporting all the world's dictatorships."

Differences with Washington have been particularly sharp on the issue of the conflict in El Salvador, whose leftist guerrilla organization France and Mexico recently recognized as a necessary partner to negotiations to end the civil war. Washington, which has emphasized Cuban and Soviet support for the guerrillas, is aiding the ruling junta.

"French policy considers that on this Central American question the Americans have a policy that is disputable and simplistic," Mitterrand said.

Although President Reagan pronounced himself pleasantly surprised by Mitterrand's firmness on the Soviet Union when the two met in Ottawa this summer, the new French government clearly sees foreign affairs, and particularly the Third World, through a different prism. Mitterrand drew attention to this in a speech last week opening a U.N. conference here on the world's poorest countries.

While U.S. representatives urged restraint by aid-seeking countries, Mitterrand called for a substantial rise in donations by wealthy nations, pledging France to increase its own aid sharply. In a thinly disguised allusion to the Reagan administration's aid cuts and reluctance to get involved in the North-South debate on development, he added: "Helping the Third World is helping yourself pull out of the economic crisis."

In earlier moves on South Africa, France three weeks ago sought to organize a diplomatic protest by Western embassies in Pretoria against the eviction of squatters around Cape Town, but was thwarted by the U.S. Embassy's refusal to take part. Similarly, France voted Aug. 31 in favor of a U.N. Security Council condemnation of South Africa for its recent attacks into Angola, a resolution blocked by a U.S. veto.

The South African Embassy here said it has no comment on the decision to allow SWAPO and African National Congress offices in Paris, which a spokesman said he learned of only today.