Catalan leader Carles Puigdemont called Saturday for "mediation" to resolve the "serious" conflict pitting his executive against the central government, on the eve of an independence referendum banned by Madrid.

"If the yes wins, if the no wins -- in any scenario there must be mediation, because things aren't working," he told AFP.

Vowing that he and his supporters would vote on Sunday, and that the regional government had everything in place so that the referendum "takes place normally," he called on Catalans to maintain a "peaceful attitude."

The standoff between the central government and Catalan leaders over their plans to hold an independence referendum has morphed into one of the biggest crises to hit Spain in decades.

Madrid argues the vote is illegal as the courts have ruled it unconstitutional.

But Puigdemont and his team insist Catalonia has a democratic right to decide on its future.

Opinion polls show the wealthy, northeastern region is deeply divided over independence, but a large majority of Catalans want to be able to settle the matter in a legal and binding vote.

Puigdemont did not call on any specific person, organisation or country to mediate, but he hinted that the EU may be well placed to do so.

"I think that from now it would be logical for the EU to actively monitor (the situation) and actively take an interest," he said.

"If it doesn't take an interest in what is happening in Catalonia when everyone is watching and taking an interest, there's something wrong."

He reiterated his call on the Spanish government to negotiate over holding a legal, binding referendum, and promised to call the entire vote off if the state agreed.

But this is unlikely to happen.

Thousands of police have been deployed to Catalonia to stop the vote, millions of ballot papers seized, key referendum organisers detained, and websites promoting the vote shut down.

But still separatist leaders are determined to hold a vote with a semblance of legitimacy on Sunday.