Spain's paramilitary national police force said on Sunday that its agents had seized more than 1.3 million posters, flyers and pamphlets promoting Catalonia's planned secession referendum.

Watch video 00:34 Catalan parliament separatists sing

The Civil Guard announced that the material was confiscated during a raid on an unnamed business that distributes advertising material in Barcelona province.

The seizure was the largest of several that have been carried out by police, bringing the total number of confiscated referendum material to 1.5 million pieces.

Also Sunday, Spain's chief public prosecutor Jose Manuel Maza warned that Carles Puigdemont, the president of the Catalonian regional government, risks being arrested depending on how events unfold.

"I won't rule out completely the option of seeking jail terms... It could happen under certain circumstances," Maza was quoted saying in Sunday's El Mundo newspaper.

Spanish state media, RTVE, put out a tweet indicating the shutdown of at least one website providing information about how to participate in the referendum.

In response, Puigdemont issued a tweet saying that referendum cannot be stopped. He also provided information about how to circumvent the block by using a web proxy.

About vote, not independence

More than 700 mayors from Catalonia gathered in Barcelona on Saturday in defiance of Spain's central government to show their support for an independence referendum.

Tensions are heating up ahead of the vote scheduled for October 1 despite opposition from Madrid and a Constitutional Court ruling calling it illegal.

Hundreds of regional mayors face arrest if they do not comply with prosecutors' orders to come in for questioning for helping to prepare the vote.

More than two-thirds of the autonomous region's mayors have said they will facilitate the vote and allow municipal buildings to be used, in violation of Madrid's orders. Organizing the referendum will be nearly impossible without the cooperation of local municipalities.

Hundreds of Catalan mayors took part in a rally in Barcelona on Saturday

The mayors attended the rally in Barcelona with the city's mayor Ada Colau and Puigdemont.

"We will not be intimidated. This is not about independence, it's about our rights," said Colau.

Colau has said she reached agreement with the regional government to allow the vote in Barcelona "without putting institutions or public workers at risk." It is unclear what arrangement was reached.

A minority of Catalans want independence, but a majority support an independence vote to settle the issue, according to a July poll.

Madrid applies pressure

On Saturday, Prime Minister Mariano Rajoy vowed to block the vote.

"The only thing I ask of mayors is that they comply with the law, and as such don't participate in an illegal referendum," Rajoy said.

Police in recent days have conduced raids seizing printing materials, ballots and other materials to support the referendum. Catalonia's top court has also warned seven newspapers against publishing campaign material.

According to July poll, only 41 percent of Catalans favor independence but nearly 70 want a vote

On Friday, the Spanish government threatened to take away Catalonia's budget to prevent spending on the referendum. Since July, Madrid has demanded weekly spending reports in an attempt to prevent public money being used for the referendum.

Spain's 17 regions pay taxes to the central government which then redistributes funds for local social security, education, police and emergency services. Budget Minister Cristobal Montoro said on Friday that the central government would take over the payments of the region's essential services unless Puigdemont guaranteed public cash was not being used to fund the referendum.

cw, bik/kl (AFP, AP, Reuters)