Francisco Labastida Ochoa, a former state governor and cabinet minister seen as the favored candidate of Mexico's ruling party establishment, won a landslide victory in the country's first presidential primary Sunday, according to preliminary official returns and independent exit polls.

The win makes Labastida the immediate front-runner in the July 2000 general election, provided the ruling Institutional Revolutionary Party (PRI) does not fracture in the aftermath of a divisive, negative primary campaign. After decades of allowing sitting presidents to handpick their successors in private, the party adopted the primary as part of an effort to revive its tarnished image and extend its 70 years of uninterrupted rule in the face of growing opposition.

With 58 percent of the vote counted early today, PRI officials said Labastida was winning in 272 of 300 electoral districts over challenger Roberto Madrazo, who portrayed himself as a party maverick and attacked Labastida as a creature of an obsolete, anti-democratic political machine. Party officials said Madrazo was leading in 21 districts. An independent survey of voters in 5,600 polling locations by the National Chamber of Radio and Television Industries showed Labastida the winner in 270 districts, compared with 22 for Madrazo. "We're winning," Labastida said in a victory speech before boisterous supporters this morning. "Today is a new PRI, born of the power of the vote."

Madrazo campaign officials disputed those results and insisted that their candidate had won at least 204 districts. The winner of the primary will be determined not by a direct popular vote but on the basis of who receives the most votes in each of Mexico's 300 congressional districts.

The conflicting vote tally is precisely the sort of dispute that the PRI had hoped to avoid. Irreconcilable claims by the two campaigns could lead to charges of vote rigging and the loser's defection from the party. It could also reinforce the PRI's reputation for electoral fraud, a legacy that has driven voters to opposition parties in recent years and that the primary was designed to repair.

"The stronger we are inside, the better for us," said Alejandro Valenzuela, Labastida's adviser for international affairs. "The weaker we are, the more difficult it's going to be for us to struggle for power next year."

The PRI primary was open to all eligible voters, regardless of party affiliation, but turnout was sporadic. Some potential voters expressed confusion about the concept of a primary, and PRI officials accused opposition party workers of using loudspeakers and leaflets to dissuade people from participating. President Ernesto Zedillo, who was widely seen to be supporting Labastida, had embraced the primary in response to calls for greater democracy and the growing strength of opposition parties in state and local elections.

In addition to Labastida, 57, and Madrazo, 47, two other candidates competed for the nomination: former Puebla state governor Manuel Bartlett, 63, and former PRI president Humberto Roque Villanueva, 55.

If the party holds together and unites behind Labastida, many analysts believe the PRI, the party of every Mexican president since 1929 and the world's longest-governing party still in power, could continue its winning streak next year in the election to replace Zedillo, who cannot run for reelection.

But if one of the primary losers bolts from the party, if the race is marred by credible complaints of fraud or if the fractured opposition parties unexpectedly unite, polls show that the PRI could lose next year.

Recent surveys indicate that the PRI nominee can expect a strong challenge from former Guanajuato state governor Vicente Fox, the candidate of the center-right National Action Party (PAN). The other main contender, former Mexico City mayor Cuauhtemoc Cardenas of the leftist Democratic Revolutionary Party (PRD), trails badly in polls.

While the PRI is conducting the most open candidate selection process of the three major parties, its long history of ballot-box fraud and election rigging raised doubts about whether the party could run a clean primary. Because Mexico's new election laws have not kept pace with the transformation of the political system, the country's independent election commission did not oversee Sunday's primary, and many international observer groups declined to monitor it.

"We are the first who want it to be totally pristine," PRI President Jose Antonio Gonzalez Fernandez said in an interview. "If there is a failure, the future of the PRI will be at stake."

Each of the PRI candidates reported minor incidents of voting irregularities at a few of the country's 64,900 polling stations, although none of the candidates alleged any serious breaches of electoral ethics. Madrazo officials accused the Labastida campaign of blocking access to some polling stations in the vote-critical state of Mexico.

PRI officials said about 6 million of Mexico's 55 million registered voters cast ballots. Despite distrust of the system, many voters such as Laura Lara, a 43-year-old saleswoman, said participating was important. "I think it's part of the PRI's strategy for the candidates to attack each other. They already knew who would win," said Lara, who cast her ballot at a fast food stand in Mexico City.

Both Labastida, the former governor of the northern state of Sinaloa, and Madrazo, who is on leave as governor of the southern state of Tabasco, ran well in their regions, according to exit polls. That left a narrow, voter-rich belt across central Mexico as the main battleground of the campaign. Mexico City, the state of Mexico that surrounds it, and the Gulf Coast state of Veracruz contain 30 percent of the voting districts.

In another key primary, exit polls and preliminary vote counts indicated that former finance secretary and ex-ambassador to the United States Jesus Silva Herzog was the winner in the PRI nomination for mayor of Mexico City, considered the second most powerful elected position in the country. The other two parties have not yet selected their mayoral candidates.

The first presidential primary in Mexican history revolutionized electioneering here. Candidates took to the airwaves, and, with prominent U.S. political consultants and pollsters as advisers, individual personalities and public image-making overshadowed traditional attributes of party service and loyalty.

Madrazo rocketed ahead of Labastida in opinion polls in August when he launched ads attacking the incestuous and corrupt machinery of his own party, despite charges that he himself had engaged in electoral fraud to win the Tabasco governor's race in 1994.

That played well in a country where many have become fed up with the autocratic power of the party and its traditional leaders.

"I voted for Madrazo because he's said a lot of things nobody else has had the courage to say," said Amparo Martinez, 42, a single mother of two who works as a maid and voted in the chilly courtyard of the high school in the poor Mexico City neighborhood where she lives.

But surveys of potential voters showed that many people tired of the hard-hitting attacks. Labastida, whose campaign rebounded from a sluggish and disorganized start, moved ahead of Madrazo in the polls in recent weeks.

Labastida, who had the backing of most of the PRI establishment, was governor of the northern state of Sinaloa from 1986-92. After he left office his state attorney general was murdered, allegedly in retribution for fighting drug traffickers. The government dispatched Labastida to Portugal as ambassador, supposedly to help protect his life.

He has been a government functionary for most of his career, holding three cabinet positions, including most recently the powerful post of interior minister in the Zedillo administration.

CAPTION: Indigenous women carrying their children wait to vote in Chiapas state.

CAPTION: Francisco Labastida Ochoa won Mexico's first primary vote.