Belgian investigators searched offices at the European Parliament in Brussels on Monday, in a probe into alleged bribes from World Cup host Qatar.
It was the 20th search carried out in Belgium since Friday and the parliament president, Roberta Metsola, warned MEPs that “democracy is under attack”.
Belgian police have arrested four people, among them European Parliament Vice-President Eva Kaili, a Greek MEP.
Qatar has denied any wrongdoing. MEPs will debate the scandal on Tuesday.
Belgian prosecutors said cash worth about €600,000 ($632,000; £515,000) was found at the home of one suspect, €150,000 at the flat of an MEP and several hundred thousand euros in a suitcase in a Brussels hotel room. Computers and mobile phones were also taken.
Since Friday, the IT resources of 10 parliamentary employees have been “frozen” to prevent the disappearance of data necessary for the investigation.
The four suspects arrested by Belgian police have been charged with “participation in a criminal organisation, money laundering and corruption”, prosecutors said in a statement on Sunday.
Ms Kaili – an MEP for eight years – was suspended from her duties as one of 14 vice-presidents by parliament president Metsola.
She has also been suspended from the parliament’s Socialists and Democrats Group and expelled from the Greek centre-left Pasok party.
Prosecutors in Greece have reportedly frozen all of Ms Kaili’s assets.
Six people were detained on Friday as part of the investigation into allegations that Qatar bribed EU officials to win influence. Two were later released.
No suspects have been publicly named, but Ms Kaili is understood to be among those who have been indicted.
The three other accused are all Italian citizens and her partner Francesco Giorgi, who is a parliamentary assistant, is among those reportedly charged.
The BBC’s Brussels correspondent, Jessica Parker, says details released by Belgian authorities in the last few days left many people’s jaws on the floor in EU circles.
MEPs who spoke to our correspondent say they are shocked by both the scale and blatancy of the accusations.
‘European democracy under attack’
European Commission President Ursula von der Leyen said the accusations were “very serious” and called for the creation of a new ethics body to oversee the bloc.
Ms Metsola vowed to MEPs that the parliament’s integrity would be restored.
“European democracy is under attack and our free and democratic societies are under attack,” she said.
Watchdogs and MEPs said the bribery investigation could represent one of the biggest corruption scandals in the parliament’s history.
Prosecutors said they suspected a Gulf state had been influencing economic and political decisions of the parliament for several months, especially by targeting aides.
Local media has named the state as Qatar, although the Qatari government said any claims of misconduct were “gravely misinformed”.
Ms Kaili’s responsibilities as vice-president include the Middle East. She has been a defender of Qatar in the past.
The European Parliament is the EU’s only directly elected institution. Some 705 members of parliament, elected by voters in the 27 nations which make up the EU, meet to scrutinise proposed legislation and vote through European law.
MEPs generally enjoy immunity from prosecution, but not in cases where “a member is found in the act of committing an offence”, the parliament says.