At the European Parliament, Greek MEP Eva Kaili denied involvement in an alleged bribery scam involving World Cup host Qatar.
She is one of four persons charged after Belgian authorities discovered substantial sums of cash in apartments and luggage.
MEPs have voted – by 625 to one – to strip Ms Kaili of her role as one of its 14 vice-presidents.
Parliament leader Roberta Metsola has spoken of “difficult days for European democracy”.
Qatar has refuted all allegations of misconduct.
“[Eva Kaili] maintains her innocence and has nothing to do with Qatari bribery,” her lawyer Michalis Dimitrakopoulos said on Greek television on Tuesday.
Prosecutors carried out a series of searches over many days and said cash worth approximately €600,000 ($632,000; £515,000) was discovered at the residence of one suspect, €150,000 at the flat of an MEP, and several hundred thousand euros in a suitcase in a Brussels hotel room.
Belgian police released a photo on Tuesday showing piles of €200, €50, €20, and €10 bills. According to reports, it amounted to €1.5 million (£1.3 million).
Asked if money had been found at Ms Kaili’s apartment, her lawyer said: “I have no idea if any money was found or how much was found.”
The suspects arrested by Belgian police have been charged with “participation in a criminal organisation, money laundering and corruption”, prosecutors said on Sunday. They will appear before a pre-trial court on Wednesday.
The three others are believed to be Italians, including Pier Antonio Panzeri, a former MEP who currently runs a human rights organization in Italy.
The charges have put doubt on the role of lobbying organisations in the European Parliament. MEPs were expected to vote this week on a recommendation to enable Qataris visa-free travel to the EU, but it has already been withdrawn.
Searches have been conducted in both Italy and Brussels. Since Friday, the IT resources of ten parliamentary employees have been “frozen” to avoid the removal of data required for the investigation.
Parliament, sitting in Strasbourg, voted overwhelmingly on Tuesday to strip Ms Kaili of her role as vice-president.
She has also been suspended from the parliament’s Socialists and Democrats Group and expelled from the Greek centre-left Pasok party.
Greek authorities have frozen assets belonging to her, her husband and immediate family members.
The activities of a property firm established around two weeks ago in Athens by the MEP and her husband have also been curtailed.
On Friday, six persons were held as part of an investigation into suspicions that Qatar bribed EU officials in order to gain influence. Two of them were later released.
The three other defendants are all Italian citizens. Francesco Giorgi, Ms Kaili’s parliamentary assistant, is allegedly among those charged.
According to sources, former MEP Pier Antonio Panzeri, who currently manages the human rights organization Fight Impunity, is also among the four. His wife and daughter were reportedly arrested as well.
According to the Italian news agency Ansa, the other suspect is Niccolo Figa-Talamanca, who manages the lobbying group No Peace Without Justice.
Luca Visentini, the head of the International Trade Union Confederation, was questioned and later released. He then claimed that he was able to answer all queries posed to him.
According to BBC Brussels journalist Jessica Parker, data disclosed by Belgian officials in the previous few days have left many people in EU circles stunned.
MEPs who spoke to our correspondent expressed disbelief at the magnitude and bluntness of the claims.
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 said ahead of the vote to strip Ms Kaili of her leading role at the Parliament that “European democracy is under attack and our free and democratic societies are under attack”. She and the Parliament would do everything they could to fight corruption, she added on Tuesday: “There will be no impunity, there will be no sweeping under the carpet.”
According to watchdogs and MEPs, the bribery investigation could be one of the biggest in the history of the parliament.
Prosecutors said they suspected a Gulf state had been influencing the parliament’s economic and political decisions for several months, particularly by targeting aides.
The state has been identified by local media as Qatar, while the Qatari government has stated that any allegations of misbehavior are “gravely misguided.”
As vice president, Ms Kaili was in charge of the Middle East.
While her colleagues were highlighting Qatar’s human rights violations, she gave an exuberant speech to parliament last month hailing Qatar for hosting the World Cup and for being a “frontrunner in labor rights.”
Green MEP Daniel Freund, who heads a cross-party anti-corruption group, told the BBC that the parliament had “reasonably good lobby transparency rules” but that third countries like Qatar were excluded from them: “So I think these third countries should go on the lobby register.”
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.
