A national group of labor, civil rights, education and religious leaders yesterday accused the Mexican government of hiding rampant political repression behind its international image as a liberal Latin state.

The Council on Hemispheric Affairs (COHAS) also accused the U.S. government of condoning the alleged political repression by denying asylum to Mexican nationals who claim they are the targets of political persecution in their country.

"There is... a reluctance on the part of Washington to offend the Mexican government, particularly now that Mexico is a potential large-scale supplier of oil and natural gas to this country," said COHAS director Larry Birns.

"We realize the importance of continued good relations with Mexico... But we feel such flagrant violations... transcend political or economic considerations."

At a news conference yesterday, COHAS accused the Mexican government of torturing, wrongfully jailing and occasionally killing political dissidents. The charges, which also have been made by human rights groups such as Amnesty International, come one week before President Carter is to travel to Mexico for a three-day meeting with President Jose Lopez Portillo.

As examples of the alleged repression, COHAS offered the cases of Hector Marroquin and Jesus Piedra, two former student activists in Mexico.

Piedra was arrested by Mexican authorities in April 1975 and has not been heard from since. His mother, Rosario Ibarra de Piedra from Monterrey in northern Mexico, has been leading an international campaign to find out the whereabouts of her son and other student activists who disappeared under similar circumstances.

Marroquin fled to the United States in April 1974 after the Mexican government accused him of robbery and murder in connection with his political activities. He has steadfastly denied the charges and is seeking political asylum in the United States on grounds he faces probable death upon his return to Mexico.

Last Dec. 21, the Immigration and Naturalization Service denied Marroquin's request for political asylum. The case has become an international cause celebre.

"Human rights, to them [American officials], don't exist for me because the U.S. is friendly with the country I come from, and because of my political beliefs," Marroquin, an avowed socialist, said at yesterday's press conference, which also was attended by Mrs. Piedra.

The INS said it rejected Marroquin's asylum request because he failed to establish that he is likely to be persecuted in Mexico because of his political beliefs.

"We made that decision solely on the basis of the facts and the information before us," an INS spokesman said yesterday.

Marroquin is scheduled to appeal the ruling April 3. Besides COHAS, he is supported in that endeavor by Amnesty International, the National Education Association and Reps. Ronald Dellums (D-Calif.) and Parren J. Mitchell (D-Md.), members of the Congressional Black Caucus.

Dellums said yesterday that Morroquin's "real crime is that he had the moral courage to speak out for human rights" against a government that is using "institutionalized terror and violence masquerading as law" to put down dissent.