Mexico's ruling party claimed victory in elections for seven state governors and a new lower house of Congress today in balloting marred by hundreds of reported incidents of voter intimidation, ballot-box stuffing and other irregularities.

A spokesman for the Institutional Revolutionary Party (PRI) said tonight that based on preliminary returns, the party had won all seven governorships and all 300 congressional seats.

But gubernatorial candidate Fernando Canales Clariond of the opposition National Action Party charged the government with committing "flagrant" violations of Mexican electoral laws in at least 200 of the state of Nuevo Leon's 1,500 voting places. In the evening he led a demonstration of thousands of his supporters in front of the state house demanding that the government stop resorting to what his party claimed was widesprad fraud, voter intimidation and open theft of ballots.

As the polls closed tonight there were 29 independent reports of ballot boxes being forcibly removed from polling places either by officials or by plain-clothed gunmen. In one neighborhood, Colonia Moderna, four men carrying pistols and machine guns broke into the polling station after it closed and took away the ballot-filled boxes, residents and reporters said.

In another incident, in the Colonia 16 de Septiembre on the outskirts of the city, National Action poll watchers were forced against the wall by police as the vote count began. As residents protested, about 20 additional municipal police arrived and escorted governing party officials and the ballots to a waiting station wagon. The car sped off with opposition party supporters in pursuit.

There were reports of clashes between opposition supporters and police and Army troops in other border states.

Today's election marked the halfway point and most important electoral test of President Miguel de la Madrid's six-year term. The PRI had to fight hard to beat back opposition challenges in gubernatorial contests in Nuevo Leon and Sonora states as well as in several dozen of the country's 300 congressional districts. The PRI, founded 56 years ago, has never lost a state or national election.

Despite the intense summer heat, voting was moderate to heavy in the seven states holding gubernatorial races. Turnout was reported lighter in the rest of the country. National congressional elections, held every three years, usually draw about half the electorate, in contrast to the turnout of more than 65 percent common in Mexico's presidential elections.

The governing party claimed a 4-to-1 victory in Nuevo Leon state, where the standard bearer for governor was Jorge Trevino. His opponent, Canales said that what he called wide-spread fraud had made it impoossible for his party to estimate the results of today's voting. Final, official results for the state and national elections will not be known until next Sunday.

Many observers had asserted before the balloting that the progovernment vote would be inflated by fraudulent voting.

"The use of fraud is so ingrained here that the party can't break the habit," said Javier Livas Cantu, a former local PRI official and son of a onetime PRI Nuevo Leon governor. Backed by the Monterrey daily El Porvenir, Livas Cantu and several fellow PRI dissidents have organized a sophisticated computer center to monitor voting results here. The center's data base will be the copies of voting booth reports that are given to opposition poll watchers but that Livas and other experienced observers say are often altered.

Moreover, computer monitoring cannot detect multiple voting, an increasingly institutionalized practice here, many politically unaffiliated analysts say. Several members of progovernment unions today visited the office of El Norte, a local paper that provides extensive opposition coverage, and anonymously turned over thick wads of premarked ballots that they said they had been "ordered" to insert in their local voting boxes.

In the northwestern state of Sonora, where PRI candidate Rodolfo Felix Valdes faced Adalberto Rosas of National Action, opposition leaders accused the PRI state government of disqualifying legally registered National Action poll watchers and permitting multiple voting by government supporters.

"We are used to these tricks, but today we are seeing double or triple the amount of fraud that we have before," declared Teresa Garcia de Madero, a National Action candidate for the Nuevo Leon legislature.

In one voting booth visited in the unpaved back streets of Garcia de Madero's working-class district, voters who did not present registration cards, as the law requires, were handed ballots. A bottle of ink, intended to mark the thumbs of departing voters as a legal safeguard against duplicate voting, stood unopened in a corner. Also unopened was the thick voter registration list.

The president of the committee overseeing the polling place, Joaquin Munoz, had no record of who or how many people had voted. "In any case, we aren't obliged to tell anyone anything," he said.

In another Monterrey neighborhood, Colonia Alvaro Obregon, voting committee president Tomasa Ramirez was accused by a National Action representative of letting children vote while systematically blocking registered voters believed to be opposition sympathizers. When journalists arrived, she seized one reporter's government credentials and called police, demanding that the reporter be arrested "for interfering in local politics."