FIFA has launched an investigation into why there were around 3,500 empty seats at Friday's opening World Cup match when Senegal produced one of the biggest upsets of the tournament's history by beating France.

FIFA communications director Keith Cooper told a news conference yesterday that 62,651 spectators attended the 1-0 defeat of the world champions, which was played in a superb atmosphere, saying the governing body was "very satisfied" with the organisational aspects of the match.

But he added: "There was noticeable space, especially in one corner. There was a total of 3,500 unused seats. That is now the subject of a study. We hope by tomorrow to have more information."

Empty seats in a stadium at football's most prestigious tournament provide disappointing pictures for television. Friday's game in Seoul was estimated to have been watched by 500 million people worldwide.

Cooper said one of the reasons may have been that a contingency of seats had been kept vacant in case there were problems with the merger of two ticket databases.

Officials had feared there might have been duplication of tickets and wanted to keep some seats aside to make sure fans who had bought tickets were guaranteed a place.

But he said only 100 duplicate tickets had been identified. "One hundred out of 3,500 is not many so that is the reason for the investigation," he said.

Around 100,000 tickets have yet to be collected from the stadiums for the tournament, most of them in Japan, though FIFA says this involves between 2,000 to 3,000 tickets at each game which the governing body believes is "not unusual" for football.

Around 11,500 security personnel were working in and around the Seoul World Cup stadium on Friday, 119 of whom were responsible for monitoring air pollution levels, Cooper said.

Opening games can be disappointing but Senegal's surprise victory has added spice to the first finals to be held in Asia, which need to be entertaining to attract television audiences in the key market in Europe where matches are largely broadcast in the morning because of the time difference.

Asked if the game provided the perfect start to the finals, Cooper said: "I am not sure what the perfect start is but it was as close as you can get to it.

"I am not a great fan of opening ceremonies and there is a feeling in the organisation that they can get in the way of the match. But this was impeccably organised," he said.

"The result has got the world champions with their backs to the wall, having to win their remaining two group games. One group is already standing on its head."