Hundreds of thousands of people have staged a rally in Barcelona to show support for Catalonia’s separation from Spain in the upcoming independence referendum.

An estimated 400,000 Catalans from all walks of life marched through central Barcelona on Monday, waving red, yellow and blue separatist flags in a show of strength three weeks ahead of a breakaway referendum declared as “illegal” by Madrid.

The rally coincided with Catalonia�s national day, which commemorates the fall of Barcelona to Spain in 1714 and is traditionally used by pro-independence activists to call for secession.

"If there is huge mobilization, they can�t do anything in Madrid," says Jordi Calatayud, a 21-year-old economics student attending the rally. "Catalan people will make independence possible, if there are a lot of us, they can�t stop us."

"We hope that we will be able to hold the referendum with total normality, because in a democracy it is normal to be able to vote," another participant said. "If the people want it to happen, it will go ahead."

People wave �Esteladas� (pro-independence Catalan flags) as they gather during a pro-independence demonstration, on September 11, 2017 in Barcelona. (Photo by AFP)

Last Thursday, Spain’s Constitutional Court issued a ruling to suspend the referendum after Prime Minister Mariano Rajoy legally challenged the vote and said Spain was constitutionally indivisible.

Regional leaders, however, have vowed to go ahead with the vote despite criticism from Madrid, which has threatened to disqualify the head of Catalonia�s regional government, Carles Puigdemont, who is facing criminal charges of misuse of public money, disobedience and abuse of office for organizing the referendum.

The Catalan leader says the Spanish government does not have the authority to do so and insists that the vote will proceed at any cost on October 1st.

"It�s not an option that the referendum won�t go ahead," he told reporters. "It�s twenty days away and we�ve already overcome many hurdles."

President of the Catalan Government Carles Puigdemont answers to media after a pro-independence demonstration, on September 11, 2017 in Barcelona. (Photo by AFP)

Catalonia, a region of 7.5 million people with its own language and culture that accounts for about one-fifth of Spain�s economic output, has significant powers over matters such as education, healthcare and welfare.

But Spain�s economic doldrums and a perception that the region pays more in taxes than it receives in investments and transfers from Madrid have helped push the cause of secession from the fringes of Catalan politics to center-stage.

Recent polls by the regional government suggest that those opposing independence outnumber the supporters by a low margin. However, local authorities say Catalans would decide on the issue once and for all in the upcoming referendum.