EU's foreign policy chief Josep Borrell has described arrests over alleged corruption at the European Parliament as "very, very worrisome".
Four people have been charged, while two have been released.
Minister for Foreign Affairs Simon Coveney said that the charges against four people were "very damaging".
Prosecutors believe a Gulf state, suspected to be Qatar, tried to influence parliament by donating money or gifts. Qatar has denied misconduct.
Among the arrested was European Parliament Vice President Eva Kaili, who is understood to have been charged.
Kaili, one of the European Parliament’s 14 elected vice-presidents, has been suspended from the Socialists and Democrats Group, and expelled from the Greek centre-left Pasok party.
Kaili was responsible for representing parliament President Roberta Metsola in the Middle East. She has been stripped of her parliamentary responsibilities following her arrest.
Prosecutors in Greece have reportedly frozen all Kaili's assets, the BBC reports.
Belgian police seized cash worth around €600,000 in 16 searches in Brussels at the weekend.
"[The four] are charged with participation in a criminal organisation, money laundering and corruption," the Belgian federal prosecutor's office said.
Digital devices were also taken away for examination.
Parliamentarians have voiced shock at the arrests, which, according to Euronews, is linked to the detention of family members of a former Italian MEP, who were allegedly offered a holiday worth €100,000 by the Qataris.
Former MEP Michiel van Hulten, who is a director of anti-corruption organisation Transparency International EU, said: "While this may be the most egregious case of alleged corruption the European Parliament has seen in many years, it is not an isolated incident.
"Over many decades, the parliament has allowed a culture of impunity to develop, with a combination of lax financial rules and controls and a complete lack of independent – or indeed any – ethics oversight. In many ways it has become a law unto itself.”
The European Parliament is due to vote this week on a proposal to extend visa-free travel to the EU for Kuwait, Qatar, Oman and Ecuador. Some lawmakers have suggested that the debate and vote should be postponed.