"We are very surprised about the change of opinion of FIFA," European Clubs' Association (ECA) chairman Karl-Heinz Rummenigge said in a statement.
"In several meetings, we had the consensus to change the eligibility for the Olympic Games to under-21 players with no over age players.
"We think it's not acceptable and encourage FIFA to maintain its first position."
The Olympic football tournament is currently open to players aged under-23 with up to three over-age players allowed per team.
"We remember some statements from Mr Blatter supporting this proposal. We thought it was clearly accepted by FIFA," Barcelona president and ECA vice-chairman Joan Laporta told reporters.
"We expect Mr Blatter to honour his commitments."
The Beijing Games were marked by a number of club-versus-country disputes, the most publicised involving Laporta's Barcelona and Argentina striker Lionel Messi.
Barcelona eventually allowed Messi to take part after winning the case at the Court of Arbitration for Sport (CAS) and Argentina went on to win the gold medal.
"We had the right to keep the player, it was our decision to release him," said Laporta. "They won the gold medal and Messi had a marvellous season for Barcelona, winning the championship."
