The Belgian public prosecutor is investigating suspected corruption and influence in the European Parliament by a Gulf state.
On Friday there were 16 searches in Brussels and four people were arrested, the prosecutor said.
The investigation is about a suspected criminal organization and allegations of corruption and money laundering.
The public prosecutor's office did not say which Gulf state it was.
According to research by the newspaper "Le Soir" and the magazine "Knack", it is about Qatar.
According to the public prosecutor's office, the Belgian police have suspected for several months that a Gulf state is trying to influence the political and economic decisions of the European Parliament.
Substantial sums of money or gifts in kind are said to have been distributed to persons in Parliament who held a political or strategic position.
Basically, the operation on Friday was primarily aimed at employees of MPs, it said.
A former MEP was also among the respondents.
It was initially not clear whether he was also arrested.
According to the public prosecutor's office, 600,000 euros in cash and mobile phones were confiscated during the searches.
According to information from the AFP news agency, the Greek MP and Vice President of the EU Parliament Eva Kaili was also arrested in the course of the investigation.
Kaili was arrested in Brussels on Friday evening and is now being questioned by the police, according to sources familiar with the case.
The politician has been expelled from her Greek party, the Panhellenic Socialist Movement (Pasok), because of the investigation, party leader Nikos Androulakis announced on Friday evening.