Tens of thousands of demonstrators have taken to the streets of the Spanish city of Barcelona, the capital of the autonomous Catalan region, to voice their support for the Catalonia independence referendum called for October 1.

The pro-independence demonstrators convened at Montjuic hill on Sunday, waving flags and holding up placards during a rally dubbed "Referendum is Democracy."

According to city's authorities, an estimated 30,000 people took part in the march, while some separatist sources put the figure at 47,000.

The massive rally had been organized by the Catalan National Assembly (ANC), the Association of Municipalities for Independence (AMI) and the Omnium Cultural association. Manchester City FC's coach Josep 'Pep' Guardiola, a popular figure among Catalans, also appeared in front of the pro-independence supporters.

"We are here today to make it clear that on [the] 1st [of] October we are going to vote in a referendum to decide our future", Guardiola told the rally in Catalan, Spanish and English.

"We'll vote even though the Spanish state doesn't want us to. We have tried to reach an agreement 18 times and the answer has always been 'no'," he added.

The October 1, as the plebiscite date, was set by Carles Puigdemont, the leader of the regional government, on Friday, in defiance of Madrid and even though such a referendum has been ruled illegal by Spain’s Constitutional Court.

Back in November 2014, a symbolic referendum on independence was held in Catalonia. The vote, which was not binding, went ahead despite fierce opposition by the central Spanish government, which considered it illegal. More than 80 percent of the 2.3 million people who cast ballots backed independence for Catalonia, according to Catalan officials.

Catalonia’s regional government has repeatedly made attempts for gaining an outright independence from Spain, but it has failed due to a division among the 7.5 million residents of the region. The latest regional government poll shows that 48.5 percent of respondents opposed independence, with 44.3 percent in favor.

However, many in the wealthy region, which provides almost a fifth of the Spanish economic output, have been wishing greater autonomy from Madrid in recent years. They argue that Spain's recent economic downturn is making Catalans pay more taxes to the central government to subsidize poorer regions. The highly-industrialized region has its own language and customs.