MEXICO CITY, JUNE 5 -- This smoggy capital, considered the world's most polluted metropolis, played host today to World Environment Day, a United Nations-sponsored event designed to raise ecological consciousness.

After launching a nationwide tree-planting campaign Monday, President Carlos Salinas de Gortari presided today over ceremonies honoring environmental achievements and stressing his government's support for "international ecological solidarity." Public schools held special environmental workshops aimed at driving home the U.N. theme, "A Better Planet for Children," and the government organized a "reforestation telethon."

However, some leading Mexican environmentalists criticized the activities as largely cosmetic and accused the government of engaging in little more than "green rhetoric" in place of serious efforts to remedy Mexico's worsening environmental degradation.

Even as Salinas launched his campaign to plant at least 5 million trees throughout Mexico, ecologists complained that massive deforestation was continuing unabated in southern Mexico's Lacandona Forest, the largest tropical rain forest in North America. Experts who have studied the forest say it is being destroyed at a faster rate than the Amazon rain forest and is threatened with disappearance in the next five to 10 years.

In a report issued last month, a representative of the World Bank's environmental division criticized a lack of practical measures to protect the Lacandona Forest, in contrast with government pronouncements. "This absence of protection . . . lends little credibility to the much stated government resolve to promote conservation," the confidential report said.

The report's recommendations on safeguarding the forest as a condition for an environmental-protection loan were initially rejected by Mexican authorities, but they have since reconsidered, ecologists said.

Among other serious environmental problems are air pollution in Mexico City and the increasing contamination of the country's lakes and rivers. The government's "Day Without a Car" program -- aimed at reducing automobile traffic in the capital by a fifth on weekdays -- has kept the city's smog from getting much worse since the project was begun in November, ecologists said, but dangerously high levels of ozone, lead and other pollutants continue to be recorded routinely.

Homero Aridjis, president of the Group of 100, the leading environmentalist organization here, contends that the anti-smog program is gradually being "sabotaged" by Mexican authorities, who continue to issue licenses to all kinds of "junk on wheels" without regard for the pollution this causes. As the number of cars climbs in this already overcrowded capital of 20 million people, the effect of the "Day Without a Car" program will be lost, Aridjis said.

Last month, the maximum tolerable level of ozone -- 0.11 parts per million for one hour per year -- was exceeded almost every day, sometimes by double or triple the standard for as much as four to nine hours a day, Aridjis reported. In addition to ozone, a potentially cancer-causing gas, environmentalists have expressed concern about continuing high levels of lead, which can slow mental development in children.

"The government thinks that ecological problems can be solved with slogans, spots and speeches," Aridjis said in an interview. "The resources being used to improve Mexico's image abroad should be used to solve ecological problems at home." The Mexican government last week placed full-page advertisements in leading U.S. newspapers featuring a photo of Salinas under the headline: "If we don't address the issue of global ecology, we won't have to worry about the other issues." The text called World Environment Day "a proud day for Mexico."