Four people "active in the European Parliament" have been arrested amid suspicions of corruption involving "a Persian Gulf country".
Belgium's Federal Prosecutor's Office said it recovered €600,000 in cash and seized computers and mobile phones in Friday's swoop.
It said the investigation was centred on European parliamentary assistants but that one former MEP had been questioned.
"For several months, investigators of the Federal Judicial Police have suspected a Gulf country to influence the economic and political decisions of the European Parliament, this is done so by paying large sums of money or offering large gifts to third parties with a significant political and/or strategic position within the European Parliament," it said.
The prosecutor's office did not name the country at the centre of the probe. Its press statement only referred to a "Persian Gulf state".
Those arrested for questioning were born in 1955, 1969, 1971 and 1987, but no other details were given. They may be brought before the investigating judge.
Contacted by Euronews, the European Parliament's press services said that "the European Parliament does not comment on judicial proceedings".
"As always, the European Parliament fully cooperates with the national authorities in charge. The same in this specific case," they added.
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