Strasbourg (France) (AFP) - The crisis in Catalonia poses a bigger threat to the EU than Brexit, a senior MEP warned Wednesday as the European Parliament prepared to hold an emergency debate on Spain's worst political crisis in decades.

Catalonia's leader has vowed to declare independence within days, claiming a mandate from a weekend referendum which was declared illegal by Madrid and the Spanish courts and marred by violence.

Images of the police crackdown on the vote drew a vocal reaction from some MEPs, with Belgium's Philippe Lamberts, the head of the Green grouping in parliament, warning the crisis "threatened the spirit of European integration, even more than Brexit".

Several Green and far-left deputies criticised the Spanish police for their actions.

But Esteban Gonzalez Pons, an MEP from Spanish Prime Minister Mariano Rajoy's Popular Party, rejected calls for mediation, saying Spain did not need "looking after".

"Deciding is Spain should break up or stay united is a matter for Spaniards and only for Spaniards," he said.

"If today you let Spain break up with Catalonia, a domino effect will follow across the continent. Instead of a Europe of 27 we will have a non-Europe of mini-states."

The European Commission, the EU's executive arm, broke weeks of silence on Monday to call for dialogue but stressed that it still regarded the vote as an "internal matter" for Spain, drawing criticism from Catalan separatists.

Wednesday's crisis debate in parliament is due to kick off at 3pm (1300 GMT), but Catalonia dominated a morning session on preparations for a summit of EU leaders later this month.

After a proposal by the three main political groups in the European Parliament -- the conservatives, the socialists and the liberals -- the debate will consider "constitution, rule of law and fundamental rights in Spain in the light of the events in Catalonia".

This watered-down motion was preferred to a tougher motion criticising Madrid, proposed by the Greens.