Belgian police have searched European parliament offices as part of a growing investigation into alleged bribery and corruption, as senior EU leaders warned the credibility of the bloc was at stake. Belgium’s federal prosecutor announced on Monday it had carried out 20 searches since Friday, including 19 at private homes and one at the European parliament offices. Earlier in the day, Greek authorities ordered the seizure of assets belonging to an MEP implicated in the bribery investigation. Four people have been charged with participation in a criminal organisation, money laundering and corruption, Belgium’s public prosecutor announced on Sunday, as part of a major investigation into attempts by a Gulf State, named in the Belgian media as Qatar, to buy influence with large sums of money and gifts. Police have seized computers, mobile phones and €600,000 (£515,000) in cash at one home, as well as €150,000 in a flat belonging to an MEP and “several hundred thousand euros” from a Brussels hotel room, according to the public prosecutor. The IT accounts of 10 parliamentary staff have been frozen to preserve data for the investigation. The purpose of the search in the European parliament was to seize data, the prosector said. None of the suspects have been formally identified, but Belgian media have named the Greek Socialist MEP Eva Kaili as among the four arrested. MEPs are expected to vote in the next 48 hours on whether to strip Kaili, who was on Monday expelled from the parliament’s Socialists and Democrats group, of her status as one of the parliament’s 14 vice-presidents. The Guardian further understands that at least four MEPs from the Socialists and Democrats group employing parliamentary assistants subject to the Belgian police investigation have stood down from roles in the parliament while the inquiry is ongoing, at the request of the group. Prosecutors said they suspected for months that a Gulf state had sought to sway decisions at the European parliament. The anti-corruption NGO Transparency International said the affair appeared to be “a bribery and corruption scandal of epic proportions” that demanded “root and branch reform of the EU institutions’ ethics and integrity systems”. Ursula von der Leyen, the head of the European Commission, said the allegations were of “utmost concern” and raised questions about public confidence and trust in EU institutions. Germany’s foreign minister, Annalena Baerbock, said: “This is about the credibility of Europe, so this has to trigger consequences in various areas.” Kaili’s office has not responded to requests for comment and Qatar has denied the allegations. “Any association of the Qatari government with the reported claims is baseless and gravely misinformed,” an official told Reuters. The European parliament president, Roberta Metsola, on Monday said European democracy was under attack and promised an internal investigation to “look at how our systems can become yet more watertight”, including reforms for greater transparency on NGOs funded by foreign governments. “Malign actors linked to autocratic third countries have allegedly weaponised NGOs, unions, individuals, assistants, and members of the European parliament in an effort to subdue our processes,” she said. Kaili, a former TV news anchor who became an MEP in 2014, has been a strong supporter of Qatar in the European parliament. She said as the World Cup began last month that the contest showed how “sports diplomacy can achieve a historical transformation of a country” and described Qatar as a “frontrunner in labour rights”, despite longstanding concern about the deaths of thousands of migrant workers. The revelations continued to reverberate through Greece on Monday where the head of the country’s anti-laundering authority, Charalambos Vourliotis, ordered the freezing of Kaili’s assets. The instruction affected both Kaili’s real estate portfolio and bank accounts. Kaili had stated in her annual review of her assets to parliament that she owned five properties in Greece and had approximately €340,000 in Greek bank account deposits. Among Kaili’s assets is a real estate company, opened last month in the upmarket district of Kolonaki in Athens, the conservative daily Kathimerini reported on Monday. The Greek TV station Star reported that Kaili had been found in possession of cash stuffed in bags. In Athens and Thessaloniki, Kaili’s home town, there was horror that the former news presenter, who had seemingly shown such potential, should be facing allegations of such magnitude. Ioannis Karmiris, a taxi driver, admitted to being glued to his radio since the revelations broke. “They were saying on the radio today that in her position she was taking home €15,000 a month, 10 times the average Greek wage. Why would she want more? In the end, what are bag loads of bank notes worth?” The columnist Giorgos Papachristou questioned in the Ta Nea newspaper how a woman gifted “in life with everything, or almost everything, could end up in two or three days losing it all”. MEPs from the European parliament’s radical left group have called for the creation of an independent ethics body and accused Kaili’s Socialists and Democrats of watering down the text of a resolution last month condemning the deaths of migrant workers involved in building stadiums, training grounds and hotels for the Qatar World Cup. Manon Aubry, the group’s co-president, said amendments proposed by the radical left on Qatar’s “insufficient legal protection” of migrant workers had been voted down by MEPs from the Socialist group, who she blamed for “closing their eyes to human rights violations”. She also condemned the resolution’s approval of the “strategic partnership” the EU had signed with Qatar and other Gulf states. “What this affair demonstrates is the need to clean up the institutions.” Von der Leyen has been criticised by transparency campaigners for slow progress on her pledge to create such a body when she was seeking election as European Commission president in 2019.