Leaders of the EU Parliament accused foreign powers on December 12 of trying to corrupt European democracy, as a probe into alleged bribes from World Cup host Qatar widened.
Belgian investigators, meanwhile raided offices in the parliament's Brussels base, seeking to seize computer data to track the alleged web of corruption.
One of the vice presidents of the parliament, Greek socialist MEP Eva Kaili, has been charged with "criminal organisation, corruption and money laundering".
The Parliament's president, Maltese conservative Roberta Metsola, vowed to members meeting at their seat in Strasbourg on Monday that the body's integrity would be restored.
"Make no mistake," Metsola said. "The European Parliament, dear colleagues, is under attack. European democracy is under attack and our free and democratic societies are under attack."
Leaders of the body's political blocs agreed.
"We won't let our work be sullied, we won't let our parliament or our Europe be sullied," promised Stephane Sejourne of the liberal group Renew.
Manon Aubry of the left-wing GUE group dubbed the scandal the "most serious" in the parliament's history. "Our democracy is not for sale," she declared.
Searches were carried out at the homes of MEPs and their associates in Belgium and around a million euros in cash has been recovered after allegations that Qatar has been seeking to burnish its image.
Belgian prosecutors said 600,000 euros ($630,000) were 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.
'My fury, my anger '
The Gulf monarchy, which is currently hosting the football World Cup, denies any involvement. "Any claims of misconduct by the State of Qatar are gravely misinformed," a Qatar official told AFP.
Kaili has been stripped of her vice-presidential responsibilities and parliamentary group leaders will meet Tuesday to prepare a vote to formally remove her from the post.
She is remanded in custody and will face a hearing on Wednesday to see if she and three fellow accused will need to remain in detention pending a corruption trial.
European Parliament members usually have immunity from prosecution, but Belgian investigators said that was not the case this time as suspects were caught "in flagrante".
Kaili visited Qatar just ahead of the World Cup and praised the country as a "frontrunner in labour rights", a sentiment she has repeated on the floor of the parliament.
She received a new legal blow on Monday, when Greek authorities froze the 44-year-old former television presenter and her relatives' assets.
At the parliament a wide-ranging internal investigation has been launched, Metsola said, and the parliament will continue to fully cooperate with and support the Belgian authorities' probe.
Metsola, who attended in person a police search of a Belgian MEP's home on Saturday, said she would choose her words carefully to avoid jeopardising the police investigation.
"So, if my fury, my anger, my sorrow do not come across, please be assured that they are very much present," she said.
"These malign actors linked to autocratic third countries have allegedly weaponised NGOs, unions, individuals, assistants and members of the European Parliament in an effort to subvert our processes."
Kaili was a member of the Socialists and Democrats (S&D) group in the parliament. Meeting on Monday, the group's leaders voted to expel her and to suspend other S&D MEPs implicated in the investigation.
A group source told AFP that three MEPs have stepped down from committee positions pending the investigation and a fourth, Belgian socialist Marc Tarabella, has "suspended himself".
Tarabella's home was searched on Saturday, but he has not been formally charged.
EU foreign ministers, in Brussels to discuss sanctions against Iran and Russia, warned that the scandal threatens the credibility of European institutions.
'Credibility and consequences'
Several MEPs as well as transparency campaigners have called for tougher anti-graft rules.
The European Parliament often votes to criticise corruption abroad and has pushed for action against member states accused of breaking Brussels' rules, like Viktor Orban's Hungary.
But ministers such as Irish Foreign Minister Simon Coveney warned that the scandal was "damaging".
"This is really an unbelievable incident that must now be cleared up, without ifs and buts, with the full force of the law," German Foreign Minister Annalena Baerbock said.
"Because this is also and especially about Europe's credibility and consequences must follow."