The four suspects were arrested following a "major investigation" into corruption, money laundering and criminal organisation.
Eva Kaili, the MEP suspected of accepting large sums of money to lobby in favour of Qatar, did not appear as expected at a pre-trial hearing on Wednesday in Brussels.
The Greek MEP is among four people charged with participating in a criminal organisation, corruption and money laundering that were due to appear in front of a judge.
The Federal Prosecutor's Office confirmed that Kaili requested a postponement and that she will instead be heard on 22 December.
It also announced that "the pre-council chamber confirmed the pre-trial detention" of Kaili's partner, Francesco Giorgi, and former Italian MEP Pier Antonio Panzeri.
Niccolò Figà-Talamanca, secretary-general of No Peace Without Justice, meanwhile "remains in custody but under the terms of the electronic bracelet."
The press was not allowed inside the hearing at the Palais de Justice, leaving reporters to gather outside the court.
Kaili and the three other suspects were arrested as part of a "major investigation" launched by Belgian police into a suspected operation to influence the European Parliament's decision-making that involved "large sums of money" and "substantial gifts."
Two other suspects detained over the weekend by Belgian police have been released.
One of them, the prosecutor's office confirmed, was Luca Visentini, general secretary of the International Trade Union Confederation (ITUC).
Kaili's title as European Parliament vice president was revoked on Tuesday by her own colleagues, in an overwhelming show of repudiation with 625 votes in favour and just one against.
Her assets have been frozen by Greek authorities.
The MEP's lawyer in Greece, Michalis Dimitrakopoulos, has said his client is innocent and has "nothing to do with bribery from Qatar."
Since Kaili's shocking arrest on Friday, in which she was reportedly caught in the act, Belgian police have conducted over 20 searches and seized more than €1.5 million in cash.
Police have also "frozen" the IT resources of ten members of the European Parliament staff in order to prevent the disappearance of key data that might be useful for the investigation.
Several parliamentary offices in both Brussels and Strasbourg, including Kaili's, remain sealed.
Searches have also been conducted in Italy, where Panzeri's wife, Maria Dolores Colleoni, and his daughter, Silvia Panzeri, reside.
Belgium is seeking the extradition of both women, Italian news agency ANSA has reported.
As the corruption scandal continues to send shockwaves, scrutiny has increased on the S&D group, the political family in which Kaili sat as a socialist MEP.
The S&D leadership has ordered several of its members to step down from key legislative files while the investigation is ongoing.
Although Kaili was ousted from the S&D and her role as vice-president, she will for the time being maintain her title of Member of the European Parliament.
Revoking the status of a democratically-elected representative can only be done through national legislation – in this case, Greek law.
Meanwhile, the Qatari government has distanced itself from Kaili and the ensuing political storm.
"Any association of the Qatari government with the reported claims is baseless and gravely misinformed," the Qatari Mission to the European Union said in a statement.
This article has been updated to include new developments.