Police in Belgium, France, Luxembourg and Italy have raided more than 30 premises as part of a corruption probe involving European civil servants.
The Commission HQ: EU officials did not give details of the probe
The premises raided included the European Commission's offices and the European Parliament in Brussels.
The investigation is looking into the awarding of tenders for buildings to house Commission delegations abroad and the security equipment installed there.
"The presumption of innocence must prevail", a Commission spokesman said.
He added that the Commission was co-operating fully with the investigation, which was launched three years ago.
The Brussels prosecutor's office said that the EU's Olaf anti-fraud office was assisting with the investigation, as were the Italian Carabinieri, French financial police and Belgian fraud squad officers.
"The investigation involves suspected bribery of European civil servants, forming a criminal organisation, violating professional secrecy, breaches of public tender laws and forgery," said a spokesman for the prosecutor, quoted by Reuters news agency.
In 1999, a corruption scandal forced the entire European Commission to step down. The affair involved former French Prime Minister Edith Cresson who, as European Commissioner for Research, hired her private dentist as an adviser on HIV/Aids.