The march began at 17:14 local time (19:14 GMT), a time chosen as a reminder of the year 1714, when Catalonia was defeated during the War of the Spanish Succession.
People are carrying a gigantic arrowhead from the north of the Avinguda Meridiana, a major avenue in Barcelona, toward the Catalan Parliament in the Parc de la Ciutadella. The 3-mile distance has 135 sections, symbolizing the number of members of the local parliament.
This year, the demonstration is being held ahead of regional elections on September 27. Several coalitions of pro-independence parties have pledged to sponsor the organization of an official independence referendum, if they win the vote.
Catalonia has long sought independence from Spain, accusing Madrid of constraining its economic and cultural autonomy and of unfairly distributing resources.
Over 80 percent of Catalans who participated in an unofficial November 2014 vote supported secession from Spain. More than two million people out of an estimated 5.4 million eligible voters took part in the informal, non-binding ballot.
Spanish Prime Minister Mariano Rajoy has repeatedly said that Catalonia’s secession would be illegal.