Tens of thousands of demonstrators including Pep Guardiola, the revered former manager of Barcelona's football club, rallied in the city on Sunday to support the Catalonia independence referendum called for October 1.

Carles Puigdemont, leader of Catalonia's regional government, defied Madrid on Friday by setting a date for a binding vote even though the referendum has been ruled illegal by Spain's Constitutional Court.

"We will vote, even if the Spanish state doesn't want it," Guardiola told the crowd speaking in Catalan, Spanish and English.

"There is no other way; the only possible response is to vote," he added.

As Puigdemont looked on, Guardiola also spoke calling for the international community's support against "the abuses of an authoritarian state".

While Barcelona authorities estimated the turnout at around 30,000 people, a separatist source put the figure at 47,000.

"I think independence is the only solution," Ramon Fon, a retired 67-year-old at the rally, told AFP.

"I want the referendum as a first step, and if the majority shares my opinion, then to win independence," he said, the starred flag of Catalonia draped across his shoulders.

The latest regional government poll found that 73 percent of Catalans were in favour of holding a referendum similar to the one held by Scotland in 2014 -- though that one had the approval of the British government.

But the same poll found that 48.5 percent of respondents opposed independence, with 44.3 percent in favour.

In 2014, Catalonia held a non-binding vote in which more than 80 percent of those who cast a ballot chose independence, though just 2.3 million out of 6.3 million eligible voters took part.

Catalonia, a wealthy region of 7.5 million people in northeast Spain, is fiercely proud of its language and customs, and has long demanded greater autonomy from Madrid.

Puigdemont said Friday that people will be asked to vote on the question: "Do you want Catalonia to be an independent state in the form of a republic?"

If a majority votes "yes," the region's pro-independence government has said it will immediately start proceedings to separate from Spain

But the Spanish government of Prime Minister Mariano Rajoy has vowed to be just as tough this time around as it was in the 2014 vote called by Catalonia's regional president at the time, Arturo Mas, despite it being outlawed by the Constitutional Court.

Mas was later put on trial and banned from holding office for two years.

In February, the Constitutional Court ruled against the latest planned referendum and warned Catalan leaders they would face repercussions if they continued with their project.

Spain has a variety of measures available to halt Catalonia's vote, including suspending Puigdemont for disobedience and even taking control of the regional government.

In a bid to circumvent such action, the regional government has drafted a law seeking to extract Catalonia from Spain's legal system.

It is expected to present the bill in the next few weeks to the regional parliament, where pro-independence lawmakers have an absolute majority.

But this too will probably be suspended by the Constitutional Court.