Mexico’s President Calls for Calm as Colosio Is Mourned : Aftermath: Salinas closes down banks and stock market as nation grieves. Alleged assassin confesses. Clinton condemns the murder and pledges U.S. support.

President Carlos Salinas de Gortari called for calm and unity in his country Thursday as shocked, angry Mexicans mourned Luis Donaldo Colosio, the ruling party’s assassinated presidential candidate.

Thousands of grieving people filed past Colosio’s body, which lay in state at the headquarters here of the Institutional Revolutionary Party, or PRI as it is known.

After a private, afternoon service at a fashionable funeral home in the south of the city, Colosio’s remains were to be cremated today in keeping with the Mexican custom of quick burials, as bodies are generally not embalmed.

Salinas declared a national day of mourning, closing banks and the stock market, although schools remained opened.

“The loss of Luis Donaldo Colosio is the deepest call for Mexicans to remain united, to revive our consciousness as a people of great moral strength,” the president said in a televised address to the nation. “I call on all my countrymen to share their indignation with serenity and calm.”

But Wednesday’s assassination appeared to be tearing at the already loose seams that unite Mexico and the ruling party. Colosio supporters openly accused his rival and possible successor, Manuel Camacho Solis, of having a role in the candidate’s death.

In other key developments Thursday:

* Political uncertainty prevailed as Mexicans, for the first time in more than six decades, confronted the prospect of selecting a replacement presidential candidate for the historically stable ruling party. The decision may not be made until after the Easter holidays, although factions already began disputing the worthiness of various individuals.

* In Washington, President Clinton expressed his condolences, condemned the assassination and urged Mexicans to forge ahead in their efforts to improve their economy. The Administration also offered Mexico limited law enforcement and economic help, while seeking to dispel doubts about the country’s stability.

* In Tijuana, at a crowded news conference, Atty. Gen. Diego Valades said Mario Aburto Martinez, 23, an industrial mechanic, had confessed to killing Colosio on Wednesday after a campaign rally in the border city. He said nothing about a possible motive and refused to answer questions. Tests on the gun and Aburto’s hands show that he alone fired the gun, Valades said.

The investigation, including an autopsy, shows that Aburto fired two shots from a Brazilian-made Taurus .38-caliber revolver, fatally wounding Colosio in the head and abdomen. Valades said the case is not closed, adding that Salinas ordered him to find the full truth.

* International investors appeared to react calmly to the assassination and the possibility of a Camacho Solis candidacy.

Grief, Suspicion

On the day after Colosio’s assassination, questions flew about Mexico’s presidential campaign security measures, which traditionally are more lax than in the United States.

After Colosio had finished his campaign speech and walked about 30 feet into the crowd toward his vehicle, Aburto emerged from the crowd on the candidate’s right side and shot him at point-blank range in the temple, Valades said. Colosio was shot a second time in the stomach as he crumpled to the ground, but the first shot was the fatal one.

Colosio’s murder was the third significant blow this year for Mexico, a country that prides itself on political stability. His death follows a Jan. 1 peasant uprising in southern Mexico and the kidnaping earlier this month of a prominent banker. Neither matter has been resolved.

Mexicans were devastated by their country’s first major political assassination since 1928, when Alvaro Obregon, the anti-clerical president-elect, was killed by a religious fanatic.

At party headquarters Thursday, mourners like Guadalupe Guerrero, a 34-year-old widow in a faded green T-shirt, sobbed with grief. Her face swollen and anguished, Guerrero said, “I think of the loss of my own husband and I share this great pain with Mrs. Colosio.”

Throughout the capital, people were subdued, shocked at the candidate’s death.

“We are sad, because this is not the way we do things,” said Miguel Palestina, a 33-year-old computer programmer. “Elections were coming up. This should have been done with votes, not arms. Many people are discontented, tired of candidates who make promises and do not keep them. I worry that this will come down to armed revolt.”

The crowds at PRI headquarters mixed their grief with fury.

Clenching her fist, Avelina Laguna, a 52-year-old secretary from the city’s northern suburbs, angrily cried, “Unjust, inhumane men. They should throw the book at them. . . . We must demand justice.”

Other angry Colosio supporters questioned security measures in Tijuana, the largest city in Baja California, one of only three states with an opposition party governor.

Gov. Ernesto Ruffo Appel defended local and state police, noting that security for the candidate is the responsibility of Mexico’s equivalent of the Secret Service.

Tijuana provided the eight police cars the campaign requested, and state judicial police were on call, he said. “The campaign was given all the security it asked for.”

Political Uncertainty

Coming five months before the Aug. 21 federal elections, the assassination of Colosio--the 44-year-old former social development minister--raised questions about the presidential succession in a country where, for 65 years, power has passed peacefully among members of a single political party--the PRI.

“The bullet against Colosio was a bullet against the system,” said economist Rogelio Ramirez de la O. “It was against the notion that Salinas wanted to prolong a personal style of government that has been very effective but very biased against small people.”

The assassination, he said, increases the pressure for democratic reforms, even at the price of opening the political system to the point that an opposition candidate might win the presidency.

Candidates of eight other parties indefinitely suspended their presidential campaigns and joined the orderly crowds at the PRI headquarters compound, its high-rise buildings draped with Colosio banners.

Salinas said party officials had agreed “to act responsibly” without specifying what that means.

Colosio’s PRI successor probably will not be chosen until after the traditional Easter week holidays, a well-informed source said.

Popular former Mexico City Mayor Camacho Solis appeared to be the candidate most likely to succeed Colosio, although the name of campaign manager and former Education Minister Ernesto Zedillo was also mentioned.

Many observers were surprised in November when Salinas, exercising the traditional prerogative of the outgoing president, chose his protege Colosio over his longtime friend Camacho Solis, 48.

Colosio was considered a compromise candidate between Camacho Solis, an advocate of political and social reform, and Treasury Minister Pedro Aspe Armella, an inflation-fighter committed to free-market economic policies.

Since then, Camacho Solis’ status has risen as he negotiated with southern rebels. However, a peace agreement has not yet been reached.

Earlier this week, in rejecting the possibility of a maverick presidential bid challenging Colosio, Camacho Solis said that peace was more important to him than the presidency.

In addition, the peace envoy’s ongoing rivalry with Colosio has also tainted him, provoking criticisms that he lacked party discipline. He also has powerful enemies inside the PRI, many of whom blame him for Colosio’s death.

“Kill him, kill him,” the crowd shouted, as Camacho Solis arrived at Colosio’s funeral. Earlier in the day, mourners gathered at PRI headquarters chanted, “Colosio, si ; Camacho, no.”

Market Reaction

International investors reacted calmly to Colosio’s assassination and the possibility of a Camacho Solis candidacy. Mexican stocks dropped about 5% on light volume in international markets.

“The international community views Camacho favorably,” said Kristie King Etchberger, an analyst at D. A. Campbell Co., a Los Angeles brokerage with a large Mexican stock portfolio. “There was more concern about a split in the party.”

That split may be inevitable, said economist Ramirez: “The PRI has been fatally wounded. It has lost all prestige, what little it had.”

Still, investors felt reassured that the peso would remain stable.

Also Thursday, Mexico received good news when Salinas announced that the nation had joined the Organization for Economic Cooperation and Development.

Darling reported from Mexico City and Rotella from Tijuana. Susan Drommet of The Times’ Mexico City Bureau also contributed to this report.