Mexico's political opposition scored modest gains in the July 7 congressional elections but had no victories in two northern states where significant voting irregularities were reported, according to official results released today.

With Federal Electoral Commission returns released from 296 out of 300 congressional districts, the opposition National Action Party had won six seats in the Chamber of Deputies or lower house of Congress. The governing Institutional Revolutionary Party, or PRI, won all of the other seats except two, which were captured by the small Authentic Party of the Mexican Revolution. It is considered to be a PRI ally.

National Action's total of six seats was up from only one victory in the last congressional elections in 1982, but it was well below the conservative party's claim last week to have won more than 30 seats. In addition, PRI victories already have been declared in all seven state governors' races in the elections.

"We can't say that we are content with these results. They have given us only a little," National Action spokesman Gonzalo Altamirano said.

With the PRI keeping virtually all of its electoral posts, the main result of the election appears to have been a serious blemish on the current Mexican administration's proclaimed campaign to clean up corruption, according to Mexican university professors, diplomats and other political observers.

President Miguel de la Madrid repeatedly had promised fair elections, but many commentators said that fraud in the northern states of Sonora and Nuevo Leon seemed to be the most pervasive in recent memory. National Action is legally challenging the PRI's triumphs in those states and in some other races, but party officials acknowledged that such efforts generally have failed in the past.

"The government did not deliver on its promise. It could mean that de la Madrid was too weak to enforce his own desire for a clean vote," a political scientist at a Mexican college said.

Informal street interviews in Mexico City revealed much cynicism about the electoral process here, and a newspaper poll conducted before voting day showed a large majority had expected fraud.

"They just have elections so they can say that they had them, to comply with the law," a 46-year-old female fruit-and-vegetable vendor said. She voted for National Action, she said, but her husband voted for the PRI to avoid trouble getting a license for their produce stall.

The atmosphere remained tense in the north, where National Action supporters have staged demonstrations demanding new elections and protesting vote fraud. So far, protests have been scattered, and violence has been kept to a minimum.

National Action's losing gubernatorial candidate in Sonora, Adalberto Rosas, said he will seek to take office Sept. 13. He urged his followers to practice nonviolent civil disobedience.

"Don't pay taxes and ignore government orders," Rosas said. "There is no other option. Only in this way can we respond courageously to the gigantic electoral fraud perpetrated in Sonora."

National Action supporters continued to block traffic across the border with Arizona in two Sonora-state towns, Agua Prieta and Naco, party officials said. Police arrested eight opposition supporters in the Sonora city of Ciudad Obregon and said they had Molotov cocktails.