The Chairman of the Basque Nationalist Party (PNV), Andoni Ortuzar, tweeted his support for Mr. Puigdemont—"We are with you…"—and posted a longer message on Facebook demanding the release of all Catalan separatist leaders currently in jail on remand.

"…the political situation Catalonia is experiencing is not going to find a solution or a fix via court rulings or more prison, but with dialogue, politics, negotiation and, in the end, respect for the freely and democratically expressed will of Catalan citizens."

El Mundo reported on Sunday that a unnamed senior source in the PNV had said...