Ever since the 13th World Cup's opening ceremonies Saturday, millions of Mexicans have jammed the streets, parading, cheering and singing in a nonstop celebration. But until today, it was really a celebration without cause, because the home team had not yet taken the soccer field.

Today, on a gorgeous afternoon at Azteca Stadium, Mexico defeated Belgium, 2-1, in a Group B opening match as Fernando Quirarte and Hugo Sanchez scored first-half goals. Belgium, it seemed, had no choice but to lose in the face of the intoxicating surge of national pride the Mexican crowd displayed.

There are home-field advantages, and then there are home-nation advantages.

"To play at home is an incalculable edge," said Mexico's coach, Bora Milutinovic, who is from Yugoslavia. "I am not a Mexican by birth, but my heart swells when I see such support. Naturally, it puts more pressure on our players, but I don't think they would rather be elsewhere."

"My heart surged today," said Sanchez, the team's best player. "To be a Mexican player today was to have everything you could ever want from the game."

In becoming the first nation to host the World Cup twice, Mexico has exulted in its pride, much like the United States during the 1984 Summer Olympics in Los Angeles, although the World Cup may generate even more emotion than the Olympics among participating nations because soccer is unequaled for popularity worldwide.

Today, nearly 95,000 gathered at Azteca Stadium to honor and cheer the Mexican team. Before the match, throngs of fans crowded vendors, who sold umbrellas that fasten onto your head and sombreros the size of Cadillacs and the most curious-looking noisemakers this side of Boy George. They did an especially brisk business selling red, white and green national flags.

It was a marvelous fiesta atmosphere, undampened by a prematch drizzle and unaffected by the presence of thousands of soldiers standing near dozens of tanks and military trucks.

Still, as most of Mexico tried to put its best foot forward, many people here have wondered if the World Cup will have any lasting effect on a country with massive economic problems.

"The World Cup can only bring good to Mexico, but I don't know if it will bring any change," said Lopez Hidalgo, a vendor selling seat cushions outside the stadium. "My brother tells me, 'What comes after? What will be accomplished after the tourists leave?' "

Mexico's department of tourism estimates that World Cup visitors will inject $150 million into the economy. But for a country that suffered devastating earthquakes last September and has a deepening economic crisis, the Cup is unlikely to have any meaningful impact.

In the Zona Rosa, Mexico City's popular restaurant and shopping district, the two Mexicos collide, almost unnoticed. Every night, thousands of young, well-off Mexicans cruise the streets, honking their horns, waving banners and chanting, "Mexico, Mexico!" Meanwhile, the homeless beg for coins along the sidewalks, sometimes tugging on the clothes of passersby.

The government has plastered Mexico City with "Welcome" signs and run endless TV advertisements imploring residents, "Let's be friends to the tourist; he also deserves applause." But a group called Movimiento Urbano de Resistancia Popular also has plastered the city with stickers that display a slash through the "Mexico '86" logo with the words, "We Don't Want Goals; We Want Beans!"

But today, the stadium faithful turned their backs on the food and employment problems and concentrated on the Cup.

The match was quite ordinary, but the crowd made the day memorable. Because of a foulup, the starting lineups were not announced and neither national anthem was played. But just as play was about to start, the stadium crowd began singing the Mexican anthem, and the Mexican players scrambled to line up at attention, delaying the match by two minutes.

In the 22nd minute of the first half, Quirarte, a fullback, fielded a free kick from midfielder Tomas Boy about 15 yards from the goal, dribbled to his right and put the ball into the right corner of the net. In the 38th minute, Boy's corner kick deflected off several heads in front of the goal and came to Sanchez, who scored on a header from less than five yards out to make it 2-0.

Belgium's Erwin Vandenbergh quieted the crowd with a goal in the final minute of the half, but the Belgians could get no closer.

The Belgians spent much of the second half complaining about calls that went against them, and the Mexican players responded to the stadium's cheers at the end of the match by joining hands at midfield and raising their arms in unison.