The attack by a Greens deputy comes with Catalonia’s ousted leader Carles Puigdemont due in court in Brussels on Friday to argue against extradition to Spain.
Puigdemont sought refuge in Belgium after running an independence campaign that prompted the government in Madrid to impose direct rule on Catalonia and call new elections.
His legal team are set to argue that he and four separatist allies are being persecuted for their political opinions and shouldn’t be handed over.
But press reports say that Belgium’s Public Prosecutor has already made enquiries about the prison conditions the group would face if their extradition goes ahead.
Spain is said to have provided reassurances about the jail in Madrid in which Puigdemont would be incarcerated.
That prospect has further stirred passions on the streets of Barcelona, where independence supporters staged a fresh protest on Thursday night, to denounce the detention of other former Catalan separatist leaders who they see as political prisoners.
The Commission came under fire from Josep-Maria Terricabras, a Spanish Greens MEP, in a debate in Strasbourg about the recent brutal murder of a Maltese journalist.
He said the EU institutions are yet to show similar concern for the situation in Spain in relation to Catalonia's independence referendum, which saw widespread police brutality towards peaceful voters.
Subsequently, several members of Catalonia's democratically elected government have been imprisoned while others have been forced into exile. The Speaker of the Catalan Parliament has been released on bail.
Terricabras, a Vice-Chair of Parliament’s Greens/EFA group, said, “This has led to concerns that the European Commission may be applying double standards in responding to concerns about the rule of law in different member states.
“Concern and wish for dialogue in relation to Spain is not shown by the European institutions, despite having more than strong signals of breaches of the rule of law, of democracy and of fundamental rights.
“The police have brutally beaten peaceful people, the central government has sacked a democratically elected regional government, half of this government is in prison, the other half in exile. The President of Parliament was sent to prison, 700 mayors are judicially indicted. Do we need more? There is more. Why this unequal treatment?”