A coalition of 24 U.S. religious, political and labor groups yesterday assailed the Reagan administration's policies in the stalled talks to end South African control of Namibia and said if the impasse continues the United States should dismantle some of its growing ties to Pretoria.

In a study released during a Capitol Hill press conference, the coalition criticized the administration for insisting that Cuban troops stationed in Angola must be removed before an accord on Namibia (South-West Africa) can be reached.

The coalition, which one leader said represents more than 15 million Americans, includes the National Association for the Advancement of Colored People, Lutheran World Ministries, Americans for Democratic Action, American Federation of State, County and Municipal Employees, the Congressional Black Caucus, the National Urban League and the United Auto Workers. Although many of the groups have been critical of Reagan administration policies before, it is believed to be the first major coordinated effort on Namibia.

The study has three major recommendations. It urges the administration to end its insistence on the Cuban withdrawal; to adopt a firmer position with South Africa, and if the talks continue to be unsuccessful, move to "dismantle" some of its current ties to Pretoria; and if that is not successful, work with the rest of the Contact Group to have the United Nations consider sanctions against South Africa.

Angola insists the Cubans are necessary to thwart South African incursions against Namibian guerrillas, who are based in Angola, and the issue has become the latest sticking point in the long-stalled talks among South Africa, the African front-line states and the five-member Western Contact Group that includes the United States.

Randal Robinson, executive director of Transafrica, a black lobby for Africa and the Caribbean that was among the 24 groups involved in the study, said the Reagan administration "has failed in part in the talks because of its own recalcitration in putting pressure on South Africa."

As an example, he accused the administration of having advance warning of a major South African invasion of Angola in August. Robinson gave reporters papers he said were copies of the daily intelligence briefing from July 28 showing that U.S. agencies had detected a buildup of South African forces along the border and had been told by South African Foreign Minister Pik Botha that "Pretoria would feel compelled to launch a large-scale attack if its deadline of mid-August for completing" the Namibia talks was not met.

The State Department refused to comment on the intelligence matters Robinson cited. But a spokesman said the administration is convinced "that its approach offers the best chance for peaceful change in the region."

Rep. Julian Dixon (D-Calif.), chairman of the Congressional Black Caucus, announced at the press conference that the caucus is sponsoring a resolution endorsing the study's recommendations.

The coalition also criticized the U.S. policy of "constructive engagement" or maintaining good relations with South Africa to influence its behavior.

"Little that is 'constructive' has resulted from the 'engagement' with South Africa," the study said.