The opposition National Action Party today raised to 37 from 12 the number of congressional seats that it claims to have won in Sunday's elections despite widely reported irregularities at the polls.

Official results for the 300 congressional races were not to be released until Sunday, but diplomats, opposition politicians and other political observers predicted that National Action and possibly other minority parties would win a record number of seats in the Chamber of Deputies. The opposition won a sole congressional victory in the last national legislative races three years ago.

The governing Institutional Revolutionary Party or PRI was certain to retain its overwhelming majority in the Congress. It also apparently swept the seven state governorships being contested. "You're talking small potatoes for the opposition. The power is not in the deputies," a diplomat said.

The government failed to fulfill its promise to curb voting fraud in these elections, according to diplomats, opposition politicians and Mexican, U.S. and West European journalists who witnessed the voting. The elections, held in the middle of President Miguel de la Madrid's six-year term, have been viewed as a test of the administration's proclaimed goal of "moral renovation."

Independent Mexican newspapers, while reluctant to challenge the PRI directly, published accounts of irregularities. A Mexican television reporter said his own network has not reported the degree of irregularities because it is controlled by the government. The PRI has said fraud was minor and unavoidable in any electoral contest, and de la Madrid sounded defiant in brushing aside the opposition's complaints when he spoke to a peasant group yesterday.

"I am not concerned by the confused opinions of the minorities," de la Madrid said. Invoking the PRI's historical claim to be the party of Mexico's 1910 revolution, the president added that "we are not going to be frightened by strident proposals that seek to take away power from the Mexican revolution."

Senior officials of National Action, a conservative party, said at a news conference that they had copies of official poll tallies giving them "certain victories" in 37 districts. "Given the context, it was a significant advance," said Jose Gonzalez Torres, the party's representative on the national electoral commission.

The PRI claimed within hours after the polls closed to have won all 300 congressional districts, but some PRI officials have backed off from that assertion. One electoral official indicated that National Action might win two congressional districts in the state of Sonora.

In addition to the 300 deputies' seats at stake in the mid-term election, 100 additional seats in the chamber are divided among the opposition parties to ensure that they have some voice in the legislature.

The PRI holds all the seats in the Senate, and has won every presidential and gubernatorial election in its 56 years.

Abstention rates were reported to have been high, as is normal in midterm elections. An exception was the heavy voting in states where governorships were at stake.

The National Action officials said they would fight in court for annulment of the governors' elections in the two northern border states of Sonora and Nuevo Leon. Irregularities were reported to have been particularly severe in those states, which are both opposition strongholds.

It was unclear whether the degree of fraud was sufficient to have cost National Action the governors' races in Sonora and Nuevo Leon, but the PRI had padded its margin considerably, according to journalists, diplomats and opposition politicians.

Instances were reported of stuffing or confiscation of ballot boxes, double voting, expulsion of opposition poll watchers, and harassment of voters and polling officials.

Several reporters said that less respect was shown for laws and procedures in Sonora than in elections that they covered in El Salvador and Nicaragua.