BARCELONA -- Thousands of striking university students marched through Barcelona on Thursday to protest what they call an intensifying central government crackdown on Sunday's planned independence referendum in Catalonia.

The students are demanding the right to vote in the regional ballot on secession, which the Madrid-based national government insists is illegal.

Hundreds of police and other security forces have been brought into the region to keep the peace in the face of protests, and to enforce orders from Madrid for local authorities enabling the referendum to be arrested.

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Many protesters were carrying pro-independence flags and handmade banners, with slogans such as "we want to vote." The march Thursday and the strikes were called by Catalonia's main student unions.

A protester holds a sign in front of Catalan police outside the Catalan region's foreign affairs ministry building during a raid by Spanish police on government offices, in Barcelona, Spain, Sept. 20, 2017. REUTERS

Laia Ferrus, a 20-year-old student of education, said she had chosen to come out of a sense of democratic duty. She said, "It's no longer about calling for independence. It's about standing up for our basic principles and rights."

An iconic American cartoon character has been co-opted by the Catalan separatists as a symbol of what they consider a stifling response by national leaders in Madrid. Tweety Pie, aka Tweety Bird, and often known to his Looney Toons friends as just Tweety -- is suddenly a symbol of the secessionist movement in northeastern Spain.

A massive cruise ship docked in Barcelona, rented by Spanish authorities to house some of the additional police forces brought into the region, has been draped with large cloths to hide ironically cheery images of Tweety Pie and his nemesis Sylvester the cat painted in vivid color across its sides.

A ship, rented by the Spanish Interior Ministry to house National Police and Civil Guard police officers, is seen moored at Barcelona port, Sept. 24, 2017 in Barcelona, Spain. Getty

Italian cruise company Moby Lines owns the ship, but licensed the images from Warner Bros. to use the characters' familiar faces in its marketing.

"The use of Moby's Looney Tunes branded ship in Spain and any other related connection with these characters occurred entirely without our permission," Bloomberg Politics quoted Warner Bros. as saying in a written statement on Wednesday. Warner Bros. told Bloomberg that it had "asked Moby to take immediate steps to remedy the situation."

According to multiple Spanish media outlets, the huge sheets of fabric seen draped over the side of the ship this week are the government's effort to ease Warner Bros.' concerns over some of its most recognizable characters becoming tangled up in politics.

But the effort may backfire, providing independence seekers with a fresh, globally recognizable hashtag and meme to convey their indignation at what they feel is the federal government's strong-arm response to the call for a referendum.

In Spain, Tweety is known by the name "Piolin." By Thursday, with the yellow canary's face obscured on the side of the ship in Barcelona, thousands of Twitter users were mocking the Spanish government and backing the referendum using the hashtag, "FreePiolin."

The hashtag was being seen by more than 100,000 Twitter users per hour by Thursday afternoon. Even the English version, "#FreeTweety," was gaining steam.

Amid the protests on Thursday, one student group was reportedly distributing Tweety face masks for demonstrators to wear.