The former captain and coach of Barcelona, Pep Guardiola told supporters of the movement that "we have no other option but to vote."
Several thousand demonstrators, including Pep Guardiola, the former captain and coach of the city's football club took the streets in Barcelona to support the Catalonia independence referendum.
The nationalist government of Catalonia on Friday set the referendum date as October 1st even though it has been ruled illegal by Spain's Constitutional Court.
Madrid said that it would block any attempt to hold a vote as soon as the referendum announcement was formally rubber-stamped by Catalan authorities.
Under Article 155 of Spain's constitution, Madrid has the power to intervene in the running of Catalonia's regional government, forcing it to drop the vote. This could involve sending in the police or suspending the regional government's authority to rule.
"We will vote, even if the Spanish state doesn't want it," Guardiola told the crowd 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 called for the international community's support against "the abuses of an authoritarian state".
While the city authorities estimated that 30,000 people attended the rally, 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, said.
"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.
The wealthy region of 7.5 million people is fiercely proud of its language and customs, and has long demanded greater autonomy from Madrid.