Hundreds of thousands of pro-independence supporters from across Spain's northeastern region of Catalonia are expected to pour into Barcelona as unions call for a general strike on the fifth day of protests following the jailing of nine separatist leaders.
Many of those walking towards the Catalan capital on Friday were waving pro-independence flags, Reuters witnesses say.
Tensions between Madrid and Catalonia have been rising since Monday, when Spain's Supreme Court sentenced nine politicians and activists to up to 13 years in jail over their role in a failed 2017 independence bid.
The sentences set off protests across the region, with protesters at times clashing with police.
Pro-independence leaders went ahead with a 2017 referendum on independence, despite it being deemed illegal by Spanish courts, followed by a declaration that the region was breaking away from Spain.
Madrid responded by seizing control of the Catalan administration and putting the ringleaders on trial.
Major roads were blocked off across Catalonia on Friday and several main streets in Barcelona were closed to traffic in anticipation of the marches, as well as picket lines that had begun springing up, while regional trains and the city's metro were running on a reduced timetable.
Barcelona's El Prat airport, forced to cancel around 150 flights this week after it was barricaded by thousands of people, was running a usual for the most part, according to Spain's ministry of public works.
Nearly 40 flights operated by Vueling were cancelled on Friday due to the strike, the Spanish low-cost airline owned by IAG said on Twitter.
Barcelona town hall said 400 garbage containers were set ablaze during protests on Wednesday and estimated that the city had suffered damage totalling more than one million euros in two days.
Thursday began with largely peaceful protests as young people draped in Catalan flags tossed balls and skipped rope.
The mood shifted later in the night, with protesters setting fire to cafe chairs lining the fashionable Rambla de Catalunya street in the heart of Barcelona's tourist district.
Regional police said 16 people were arrested across Catalonia, while health officials said medical aid was provided to 42 people.
Australian Associated Press