Spanish police have seized more than a million pro-referendum posters and pamphlets in Catalonia, according to government officials.

Authorities claimed the 1.3 million prints, which included about 700,000 leaflets promoting a "yes" vote in the region's planned vote on independence, from a warehouse near Barcelona on Sunday.

"The proceedings were carried out during the morning of today [Sunday] and are the result of investigations carried out by the Civil Guard of Catalonia for the localisation of materials that promote the referendum suspended by the Constitutional Court," the interior ministry said in a statement.

"This is the largest intervention of this illegal material made so far. Altogether, almost one and a half million materials to promote the illegal referendum, as well as printing plates, have been used so far," it added.

Catalonia's regional government plans to hold a vote on independence on October 1 despite Spain's Constitutional Court having ruled the ballot illegal on the basis it defies the nation's constitutional decree declaring Spain indivisible.

READ MORE: Catalonia passes law for October 1 independence vote

Ada Colau, Barcelona's mayor, criticised Madrid's response to the crisis during a pro-referendum meeting attended by more than 700 mayors from across Catalonia, on Saturday.

"It's a disgrace that we have a government that is incapable of dialogue and instead dedicates itself to pursuing and intimidating mayors and the media," he said.

Spain's state prosecutor ordered a criminal investigation into the 712 Catalan mayors who have agreed to help stage the referendum on September 14.

Mariano Rajoy, Spain's prime minister, has urged people not to participate in the vote.

"If anyone urges you to go to a polling station, don't go because the referendum can't take place, it would be an absolutely illegal act," he said on September 13.

READ MORE: Spain summons Catalan mayors over independence vote

Catalonia, a region of 7.5 million people with its own language and culture, accounts for about 20 percent of Spain's economic output and has significant powers over matters such as education, healthcare and welfare.

About 49.4 percent of Catalans are against independence, while 41.1 percent are in favour, according to a poll commissioned by the Catalonian government in July.