Wildfires have devoured nearly 300,000 acres of forest and grasslands in 11 Mexican states and devastated a major national park on the fringe of the nation's capital, cloaking Mexico City in a shroud of ash for much of the past week.

The fires have broken out at an average of 56 new blazes a day, driven by high winds, the heat and drought of the annual spring dry season, and clear-burning by farmers preparing land for planting.The same phenomena are behind the worst fires in seven decades in the Amazon rain forests of Brazil, although those are far larger than the fires in Mexico.

Mexico has lost as much acreage to fires in the first three months of this year as in all of 1997, say government environmental authorities, who fear that the blazes, unchecked, could consume 2.4 million acres this year.

On the southwestern outskirts of Mexico City, hundreds of firefighters battled blazes in the Desierto de Los Leones National Park, one of the few remaining large greenswards in a valley of 22 million people. The fire was still smoldering but under control by week's end after blackening 860 acres - about one quarter of the forest and parkland.

In a city already blanketed in dry season pollutants, the smoke from the fires pushed pollution levels to within a few points of crisis conditions this week. The afternoon newspaper Ovaciones dedicated the top half of its front page one day to a single huge headline: "Black Cloud."

In the southern state of Chiapas, where many fires were started by farmers clearing their fields, smoke and fog were so thick that drivers had to use headlights on main highways in mid-morning. In some parts of Chiapas, site of a 1994 rebel uprising and a massacre of 45 people last December, firefighters said that Zapatista rebel sympathizers refused to allow them access to blazes in rebel-held locales. Those reports could not be confirmed independently.