Belgium opened the 12th World Cup soccer championship with an upset 1-0 victory over defending champion Argentina today, with Erwin Vandenbergh scoring the only goal.

It marked the first time since 1962 that a goal had been scored in the tournament's opening match. For more than half the contest it appeared that a fifth successive scoreless tie was in store. But Vandenbergh, 23, scored in the 62nd minute, chesting down a cross from Frank Vercauteren and driving the ball past goalkeeper Ubaldo Fillol.

Argentina tried desperately to tie the score, with Diego Maradona hitting the post with a free kick in the 76th minute, but could not break Belgium's defense.

Fans whistled and jeered the play and the 120,000 capacity Nou Camp Stadium in Barcelona was only two-thirds full for this opening match in the 24-nation competition. Fifty-two matches are scheduled over 29 days in 14 Spanish cities.

Argentina and Belgium are in Group Three of the championship, along with Hungary and El Salvador. Belgium now appears likely to qualify for the second phase, while Argentina needs at least a victory and a tie to have a chance of reaching the next round.

Argentina Manager Cesar Luis Menotti made no secret of the political significance of his team's performance in relation to the South Atlantic conflict with the British. "Every man has his post in this battle," he said of the Falkland Islands fighting. "We are soccer players and will fight for our country on the soccer field."

The game was blacked out in Britain, where the Sunday Observer fumed, "What sort of futile gesture is this? Do they think the nation might be downhearted by a brilliant South American display?"

The contest was preceded by a gala opening ceremony that lasted 50 minutes, during which skyrockets and 22,500 balloons soared over the stadium. When the English Cross of St. George was run up the silver pole in the stadium, the crowd reacted with boos and whistles; frenzied cheering greeted the raising of the Argentine flag.

Among those in attendance were King Juan Carlos and Queen Sofia. In his opening speech, the king said, "I trust the competition will reinforce friendship among all the nations represented here."

Spain spent $770,000 on the ceremony, which was attended by Brazilian superstar Pele and the Portuguese Eusebio, among others.

Pope John Paul II, who returned to the Vatican today after a peace-seeking visit to Argentina, was asked by a reporter to pick the winner of the Poland-Italy game Monday. Said the Polish-born pontiff: "I think it will be better to hide while it's being played."

On Monday, Brazil, favored to win the trophy for the fourth time, plays the Soviet Union in a Group Six match in Sevilla and Italy meets Group One rival Poland in Vigo.