With the aid of scores of strategically placed explosives and the expertise of U.S. demolition specialists, an earthquake-damaged apartment building collapsed at noon today in a chalky cloud of concrete dust.

The first of many such planned explosions here, today's blast represented the beginning of "an attempt to remove the obstacles blocking a return to normality" in the city, one high-level Mexican official said.

The government had repeatedly postponed the decision to proceed with the dynamiting of condemned buildings "to avoid panicking the public" and "out of respect for the familes of the dead," he said. Another two buildings in the same public housing complex are slated to be destroyed Sunday and dozens of other damaged structures will be demolished in coming months using the U.S. explosives techniques that had not been employed before in Mexico, government officials and U.S. diplomats said.

Registering 8.1 on the Richter scale and instantly toppling more than 400 buildings in the old downtown area of the capital, the Sept. 19 earthquake is now estimated by the government to have left about 9,500 dead, said the official, requesting anonymity.

Incomplete census records and the unregistered flight from the city of former residents make earthquake casualty figures "imprecise of necessity," the official said. Unpublished government estimates indicate that the final death toll might slightly exceed 10,000, "but it will not be much higher," he said.

"I don't think we will ever have an exact number," he said.

Although no official estimate of the total number of victims has yet been released, authorities have reported the positive identification of 4,800 recovered corpses. More than a thousand unidentified bodies were buried and another 3,500 people who lived or worked in the central city area remain listed as missing, officials said yesterday.

The Mexican government also has refrained from estimating the earthquake's economic impact. In a report issued last week, the U.N. Economic Commission for Latin America placed the value of the buildings and public services wrecked by the earthquake at $3.59 billion and said the earthquake's disruption of normal industrial and commercial activity had cost the Mexican economy another $425 million.

The U.N. report, the first comprehensive survey of the earthquake damage to be published here, said the bulk of the reconstruction costs, including the replacement of $1.16 billion in wrecked school and hospital facilities, would have to be borne by the government.

Despite this unanticipated budget burden, the government has disappointed its leftist and labor critics here by not demanding emergency low-cost loans or interest relief from its foreign creditors.

Private foreign banks agreed after the quake to postpone for six months the collection of $950 million in principal payments, while the International Monetary Fund gave Mexico a special $300 million disaster credit. But Mexican finance officals and international bankers agreed that the country's financial needs would be met not with concessionary loans but through fresh commercial credits secured by a new IMF-supervised austerity plan.

Opposition to the government's cautious debt negotiation strategy, which already was becoming widespread within the ranks of the ruling party, though rarely publicly voiced, was catalyzed by the earthquake into an open, broadly supported political movement, Mexican analysts say. Resentment against the burden of continued debt servicing in a period of acute recession, expressed since the quake with increasing vehemence in Congress and the press, is now widely seen as the major political challenge at the midpoint of President Miguel de la Madrid's six-year term.

So far, the administration has stood its ground. Last Wednesday, as several unions affiliated with the progovernment Labor Congress joined tens of thousands of other demonstrators marching through the city demanding a moratorium on debt payments, Finance Minister Jesus Silva Herzog warned another powerful government-linked labor group that a unilateral payments suspension would leave Mexico isolated and cripple the country's long-term economic development.

Political aftershocks are still being felt five weeks after the earthquake. Several top city officials were fired last week in apparent reprisal for the perceived disorganization of the earthquake rescue and reconstruction effort. Demands mounted for the resignation of de la Madrid's minister of ecology and urban development, who in an earlier official capacity a decade ago was responsible for the structural inspection of a major city hospital that collapsed in the quake.

And in what many observers say could prove to be the most important political consequence of the quake, opposition groups and the governing party are courting militant new citizens' associations representing the homeless and residents of damaged inner city neighborhoods. These groups already are launching a campaign for the direct election of Mexico City's mayor, now appointed by the president.