Former Catalan leader Carles Puigdemont | Tobias Schwarz/AFP via Getty Images Blocked Catalans demand to take seats in EU Parliament Lawyers for Carles Puigdemont and Antoni Comín call on the assembly ‘to take action.’

Two Catalan leaders are demanding that they be allowed to take their seats in the European Parliament after an initial court opinion found that Spain illegally imposed additional requirements for them to take office.

In a letter to Parliament President David Sassoli, lawyers for the two Catalans — Carles Puigdemont and Antoni Comín — insisted that their clients should be allowed to claim their seats in the assembly.

"We urge the European Parliament to take action," they wrote, "to defend its integrity as a democratic legislative assembly." And they added, "Our clients are entitled to take their seats on a provisional basis until their credentials have been verified and a ruling has been given on this dispute."

Puigdemont and Comín are fugitives from Spain, which is demanding their arrest in connection with Catalonia's 2017 independence referendum, which Madrid blocked with a fierce display of police force.

While Puigdemont and Comín won seats in the May European election, Spain insisted that they needed to return to Madrid to swear an oath to uphold the Spanish Constitution in order to take up their mandates.

The initial opinion by an advocate general of the Court of Justice of the European Union came in the case of a third Catalan MEP, Oriol Junqueras, who is jailed in Spain.

Some of the issues are identical in the cases, but a key difference is that Junqueras had already been arrested, convicted and sentenced by the time of the European Parliament election. Puigdemont and Comín have been living in Belgium, and are insisting that they should be protected by the limited parliamentary immunity afforded to MEPs.

A formal ruling in the Junqueras case by the CJEU is expected in the coming weeks.