Hundreds of thousands are expected to take to the streets of the Spanish city of Barcelona to declare their support for a planned referendum on Catalonia’s independence from the mainland amid efforts by the central government to block the highly-anticipated vote.

People from towns and villages across Catalonia were traveling to the regional capital Barcelona on Sunday as the city was bracing for a mass march on the ‘Diada’ day of September 11, which is the day marking Barcelona’s fall to Spain in 1714.

Campaigners hope this year’s march, a tradition often used to voice demands for an independent state, would nullify efforts by the Spanish government to block the October 1 referendum.

Regional leaders have vowed to go ahead with the vote despite criticism from Madrid that it is illegal. Spain’s Constitutional Court issued a ruling last Thursday to suspend the referendum. That came after Prime Minister Mariano Rajoy legally challenged the vote and said Spain was constitutionally indivisible.

Recent polls by Catalan’s regional government suggest those opposing independence outnumber its supporters by a low margin. However, regional leaders and the public insist the vote gives the opportunity to Catalans to decide on the issue once and for all. That comes as even a yes vote from a low-turnout referendum would authorize the Catalan leader Carles Puigdemont to declare independence from Spain within 48 hours.

Puigdemont urged all independentistas to appear in the Monday march to declare their support for the split from Spain.

It’s vital that we come out in force for this Diada for the Oct. 1 referendum to be successful ... I’ll be there, said Puigdemont on Twitter.

Puigdemont and 13 members of his cabinet were charged in Madrid on Thursday a day after the Catalan parliament approved the referendum. Spain’s state prosecutor has accused the officials of misuse of public money, disobedience and abuse of office. Criminal proceedings could also affect all civil servants, including teachers, police officers and others who contribute to the referendum in any form.

Puigdemont has assured the public that he has the logistics needed to hold the referendum, including the ballot boxes whose locations remain hidden. Most of Catalonia’s mayors have also promised to contribute to the vote by allowing the use of municipal facilities.

A similar Diada day rally in 2012, when Spain was struggling with economic hardship, attracted around one million people. A non-binding referendum of independence held that year recorded a low turnout of around 30 percent.