As Mexican police put the toll of dead or missing at more than 7,000, demolition teams today began surveying earthquake-damaged buildings, and rescue workers sprayed disinfectant on piles of rubble that could contain decaying bodies.

The discrepancy between official numbers of victims accounted for and persons still missing remained large. According to newspaper reports citing police officials and the attorney general's office, 2,822 victims in Mexico City had been accounted for, but the total of disappeared persons was officially put at 4,180. These figures were lower than many unofficial estimates. U.S. Ambassador John Gavin said Saturday he believed the total was at least 10,000.

The engineering surveys marked a first step on the long road toward reconstruction after the devastating tremor that shuddered through the Mexican capital last Thursday, knocking down hundreds of buildings and killing thousands.

Spotlighting the rebuilding effort, First Lady Nancy Reagan flew into Mexico City today with a check for $1 million in U.S. aid and a letter from the president expressing the sympathy of Americans for Mexico in its ordeal. On her brief visit, she toured hard-hit neighborhoods with her Mexican counterpart, Paloma Cordero de la Madrid.

In preparation for demolition of buildings, Mexican police and Army troops cordoned off a large portion of Mexico City's hardest hit area. The closing of traffic arteries produced even larger jams than usual as thousands of residents sought to return to work for the first time since the earthquake.

Several days were expected to go by before the first damaged buildings are blown up, diplomatic sources said. The Mexican government must decide which of the hundreds will have to be destroyed because of irreparable damage, a decision likely to involve political as well as engineering factors, they said.

Government officials so far have announced about 30 buildings definitely will have to be destroyed. Many more are expected to be put on the list, the sources said.

A U.S. demolition team arrived over the weekend to assist the Mexican government. The Mexican press reported an Israeli demolition team also was on the way.

Mexican officials coordinating arrival of foreign relief teams and supplies said a dozen nations have dispatched people or goods.

The United States, in addition to the demolition teams and initial financial help, has sent 13 teams of search dogs and their handlers to seek out survivors in collapsed buildings, the U.S. spokesman said. Another U.S. team has been using a special device equipped with laser beams and special television cameras to peek between the twisted chunks of concrete, he said.

The spraying of buildings to prevent disease from rotting bodies dramatized a dilemma facing President Miguel de La Madrid's government five days after the quake. Out of hope that people are still alive under the debris, heavy machinery has been used sparingly in clearing away the rubble.

More than a dozen persons were reported to have been recovered alive yesterday and rescue workers, including dog teams, were still seeking signs of life today despite fading hopes than any more living victims could be found.

"We want to live," headlined the afternoon newspaper Cuestion, reflecting widespread concern lest survivors be abandoned, "those who have not been rescued are crying from the rubble."

Five babies and one man were pulled alive from the rubble of two hospitals tonight and a seventh person signaled that he was alive by tapping his keys from beneath tons of rubble that once was the 12-story Juarez Hospital, United Press International reported.

Three of the babies were pulled from the ruins of the Centro Medico hospital, where one rescue worker said the babies had survived because they were in cribs that held off slabs of tumbling ceiling and formed air pockets. The other two babies were rescued from the Juarez Hospital

Underscoring the disparities between official and unofficial figures for the toll of dead and missing, a check of three collapsed buildings in a single two-block downtown area today indicated that as many as 600 persons, mostly women textile workers and seamstresses, may be trapped. Yet 400 of them apparently were not included in the latest tolls.

This kind of discovery is leading an increasing number of Mexicans and foreigners here to favor the highest estimates of 10,000 to 20,000 dead made by Gavin, among others.

The pressure to provide a reliable body count for the largest natural disaster in Mexico history is considerable, reflecting the immediate concern for possible survivors as well as previous experience and traditional aspects of national character.

Last November, official credibility suffered when an explosion of gas tanks in the installations of Pemex, the state oil company, left hundreds of people dead in a poor neighborhood on the outskirts of the city. The government's official toll of 452 was sharply disputed by the survivors and by relatives of hundreds of people who are still listed officially only as missing.

Many Mexicans say the government sought then, and may seek now, to keep the death count low.

A further concern centers around the difference between the number of recovered bodies and the total that may lie irretrievably buried under the quake ruins. If the first number turns out to be much lower than the second, it will add to already growing irritation with the slow progress of rescue efforts.

Today, as the Army began its demolition preparations, confrontations between victims' relatives and the government were reported at at least one site, the Juarez Hospital where 400 patients and nearly that many doctors and nurses are believed to be buried.

The low official body counts may also reflect the Mexican government's traditional concern with dignity and image, some Mexicans suggested. While other countries hit by disaster may use the highest possible total of victims in their appeal for international aid, Mexico has always been concerned with self-reliance in the face of the overwhelming might of its neighbor to the north.

All these issues came into play in a two-block area bordered by San Antonio Abad, Xocongo, Lorenzo Boturini and Gutierrez Najera streets. a few blocks from Mexico's main metro station, Pino Suarez.

Demolition and rescue teams have been working around the clock on the complex of buildings above the metro. But it was only this morning that official rescue crews were sent to the three buildings down the street.

About 200 women were reported Saturday to be trapped in one. A return trip today revealed that as many as 400 more, for a total of 600, may have reported for work at several different factories in the same compound Thursday -- 19 minutes before the first quake struck.

Hector Pena, a seminarian who has been helping run a church relief center, has talked to many relatives of women believed to be trapped. While some of these relatives nodded in agreement, he ticked off the names of the companies that had workshops in three collapsed buildings on the two blocks.

They included companies with the names of Anabel, Carnival, Austro-Boy, Jordache, Cacharel and Topeka. All ran a 7 a.m. shift. It was not clear whether any of them were connected with known firms of those names.

"We know that Anabel had 250 workers," Pena said. "And Topeka had between 100 and 150."

Pena said he did not know how many were employed in each of the others, but there was at least one workshop on each of at least three floors in each of the three buildings. The smallest company was believed to employ about 25 women.

If the two largest firms had 400 workers between them and the six smallest 25 each, the most conservative estimate leads to 600 women in the compound at the time of the quake, of whom 70 were brought out either alive or dead, Pena said.

Marian Hardy of Rockville, Md., arrived at the site this morning at the head of a U.S. volunteer rescue team equipped with trained dogs. She said the Mexican police asked for the team.

Hardy declined to speculate on the chances of finding any of the victims still alive. "We always have hope," she said.

The possible victims' relatives were less sanguine. Several men who did not wish to give their names said the government had given priority to its own buildings in the rescue work. "We needed cranes to lift the rubble off the top," one man said. "But the Army only came and used its equipment to help the factory owners get their machinery out. Once that was done, they left."