Belgium’s federal prosecutor’s office said the purpose of the search at the premises in Brussels was to seize computer data from 10 parliamentary staff.
A European Parliament vice president, Eva Kaili, was among four suspects arrested and charged with corruption by Belgian prosecutors.
Belgian prosecutors said that during the searches 600,000 euros ($630,000) was found at the home of one suspect, 150,000 euros at the flat of an MEP and several hundred thousand euros in a suitcase in a hotel room.
Greek socialist Kaili and the other suspects that have been charged will appear at a pre-trial chamber in Brussels on Wednesday, prosecutors said.
European Parliament members are usually covered by immunity prosecution, but that is not the case if a suspect is caught red-handed.