While thousands of mourners wept Friday night at a funeral mass for the nearly 400 known victims of last Monday's gas plant explosion here, many survivors of the blast expressed growing anger at the government's disavowal of responsibility for the disaster.

"Pemex is guilty of this tragedy, and nobody else," charged Jose Luis Hernandez Sosa, a leader of the Citizens' Participation Council community organization in San Juan Ixhuatepec, where, according to the government, 139 homes were destroyed. The blaze spread from a liquid gas distribution center owned by Petroleos Mexicanos, the state oil monopoly known commercially as Pemex.

Within hours of the series of earth-shaking explosions that devastated the gas depot and the surrounding low-income neighborhood, Pemex spokesmen told reporters that the fire had been ignited by a tank truck explosion in the adjacent installations of Uni-Gas S.A., a private gas distribution firm. This explanation was withdrawn later when witnesses told Pemex that the Uni-Gas facility had escaped the blaze virtually unscathed.

Government officials continue to insist that the fire was sparked by a possible gas leak in one of the several private gas companies located outside the Pemex plant, although no evidence has been publicly offered in support of the charge and the private firms were all largely undamaged by the fire.

Pemex director Mario Ramon Beteta, asked by Mexican television reporters if the oil monopoly planned to compensate families who lost relatives and possessions in the fire, replied that Pemex deserved indemnification as a victim of the accident.

Alfredo Sandoval, president of the Mexican Employers' Confederation, a leading private sector association, attacked Beteta's "hasty" assertion that Pemex is blameless and urged authorities to conduct a "very objective" investigation. President Miguel de la Madrid has directed the attorney general's office to determine the cause of the explosion.

The fatality toll, not including people missing and presumed dead, was said by Interior Ministry officials Friday to have reached 393, making the fire the worst industrial disaster ever recorded in Mexico.

The death count is expected to rise. Bodies are still being recovered from the ruined neighborhood, and scores of badly burned patients undergoing treatment in city hospitals are said by doctors to have only slim chances of survival.

"When you have burns over more than 60 percent of your body, that is a condition incompatible with life," Dr. Samuel Fuentes Aguirre, president of the Mexican association of burn specialists, said.

De la Madrid quickly offered government help in rebuilding destroyed homes and instructed Pemex to build a new gas distribution center beyond the urban area. He also ordered that the site of the destroyed San Juan Ixhuatepec plant, once the main domestic fuel distribution point for half the city and now a scorched wreckage of scrap steel, be turned into a park.

But the explosion prompted renewed demands for the removal of the many other Pemex oil and gas plants in the city area.

Singled out for special criticism was the 105,000 barrel-a-day Azcapotzalco refinery, located in one of the capital's most densely settled districts and considered the biggest stationary air pollution source in one of the world's most polluted major cities.

The refinery, dating from the 1930s, was recently expanded despite complaints from environmentalists and area residents. Between March 1983 and March 1984, Pemex reported spending $2.9 million upgrading the refinery and increasing its storage capacity by 230,000 barrels.

At midweek, de la Madrid ordered a Cabinet-level study of the possible relocation beyond urban areas of potentially hazardous industrial plants. But one key Cabinet member, Marcelo Javelly, minister of urban development and ecology, has already said that relocating Azcapotzalco would be too costly and suggested that Monday's disaster had led to an irrational hysteria about the safety of Pemex's urban installations.

Representatives of the major private gas companies and distributors' associations, who met privately with Pemex officials Thursday and Friday, have refused to respond to the government's accusations.

Pemex spokesman Salvador del Rio said the gas depot was "working normally" and equipped with all the safety devices "usual for such installations," but he said Pemex would not release details of the plant's fire monitoring and prevention systems.

"I have decided not to give out that information," he said in a telephone interview. "You have a right to ask for information, but we have the right not to give it to you."

The gas depot fire was set off in a chain of at least five explosions between 5:35 a.m and 7:30 a.m., the strongest of which rocketed propane storage tanks into homes a mile away and registered on a seismograph 15 miles to the south.

One U.S. gas industry analyst, who asked not to be named, said U.S. facilities are now equipped with systems of heat-triggered and pressure-sensitive emergency cutoff valves intended to prevent re- peated blasts such as the nearly two-hour series that preceded the fire here. It was not known whether the Pemex facility had such safety devices.

Several surviving residents of the neighborhood near the depot have told reporters that the flame in the plant's flare stack used to burn off vapors that otherwise could accumulate in the air around the installation was extinguished the night before the explosion and that the area smelled strongly of gas.

Jesus Delgado Zacarias, a bus driver who lived near the plant, said he noticed the flame was out when an intense gas smell awakened him three hours before the blast.

Pemex's safety record has made many Mexicans skeptical of its contention that the fire was the fault of one of the private companies. In the most serious such incident previously, a Pemex gas pipeline explosion in 1982 in Tabasco State killed 52 people and injured 21, according to press reports at the time. Last April, 11 Pemex employes were reported killed in another pipeline explosion in Tabasco, and a pipeline burst in June, also in the southeast, reportedly killing six persons.

By law, the Ministry of Commerce and Industry is charged with regularly inspecting all gas facilities for possible safety violations. But while the ministry monitors private gas companies, it simply authorizes Pemex technicians to inspect the oil monopoly's installations, a ministry official said in an interview.

"Pemex is the biggest company in Latin America," said the official, who asked not to be identified. "The ministry simply doesn't have the technical capacity to monitor all of Pemex's installations."

Industry regulations in effect since 1960 also state that "public and private gas storage plants must be located away from residential zones and densely populated areas." The depot that exploded this week was built in 1966, a Pemex official said, in an area residents say had been settled long before. Pemex has continued to build facilities in the area, completing this year a refined products storage center in an urban district about a mile away.

Monday's tragedy follows a series of problems for Pemex, including kickback scandals, union infighting and widespread charges of company indifference to the environmental damage caused by its operations.

De la Madrid and other high-ranking government officials have drawn criticism for failing to attend either the funeral ceremony or the mass burial of 272 of the victims.

The National Council of Urban Settler Movements, a leftist coordinating group for community organizations in poor city neighborhoods, said it plans to sue Pemex on behalf of the residents of San Juan Ixhuatepec.

Roberto Pola, president of the Mexico City Bar Association, said that the victims appeared to be entitled to compensation under Mexican law and should "not have to wait until the authorities determine who was to blame."