BUENOS AIRES, Oct 3 (Reuters) - Argentina plans to investigate attendance numbers at Sunday"s derby between River Plate and Boca Juniors, a judicial official said, after local media questioned whether numbers exceeded those permitted under COVID-19 rules governing fans" return. Supporters were allowed back into stadiums in Argentina on Friday after 18 months of empty arenas due to the coronavirus pandemic. Only those who had received at least one dose of the vaccine were permitted to attend games and the maximum capacity was limited to 50%. However, Argentine media questioned the numbers present inside River Plate"s Monumental stadium and a judicial official told Reuters an investigation would be launched. "People were detained for the sale of false tickets ... and the Public Ministry of Buenos Aires city is going to investigate whether the capacity was exceeded," a spokesperson for the office in charge of major events said. La Nacion, one of Argentina"s best-selling newspapers, put the attendance at 54,000 to 57,600. Sports newspaper Ole estimated there were 52,000 to 53,000 fans, saying that was "a count clearly more" than half the 72,000 capacity. "After seeing the images on TV and according to eyewitnesses, questions are inevitable," Ole wrote online. A River Plate representative did not immediately respond to Reuters requests for comment. The home side won the match 2-1 thanks to a first-half brace from Julian Alvarez. They were assisted by a red card shown to Boca"s Marcos Rojo after just 16 minutes. The result leaves River Plate top of the league, a point ahead of Talleres de Cordoba, who have a game in hand.