Masked demonstrators have clashed with police in central Barcelona as more than half a million people marched through the city on a fifth day of protests over the jailing of Catalan separatist leaders.

It was the biggest protest since Monday's explosive verdict, which had already brought tens of thousands of demonstrators on to the streets.

Police said 525,000 people joined Friday's protest, the latest in a mass show of anger after Spain's Supreme Court sentenced nine separatists to long jail terms over a banned referendum and an abortive independence declaration two years ago.

While most protesters remained peaceful, some demonstrators hurled stones and cans at police in riot gear, and dragged large rubbish bins to the middle of a main city boulevard, and set them ablaze. Police vans tried and failed to push people back onto the pavements.

Al Jazeera's Sonia Gallego reporting from Barcelona said that the protest "escalated into a full-blown riot in the city centre".

"It started from outside of the building of national police headquarters in one of the main squares here where you have hundreds of protesters who turned quite violent towards the police," Gallego said.

"They're protesting against police measures and attempting to take on the police. [The police] responded in kind with smoke bombs."

Gallego added that the city was in lockdown due to the protest's "violent turn".

"It's quite difficult to see how the police are controlling this as it's spread to part of the city centre," Gallego said.

Friday's protests took place as the Spanish Interior Ministry announced it has given the go-ahead for civil guard police reinforcements to be sent to the outskirts of Barcelona.

Security reinforcements

Interior Minister Fernando Grande-Marlaska told reporters on Thursday he planned to send security reinforcements to Barcelona to help maintain order in the city and also to let police already there get some rest.

Unions in the wealthy region called for a general strike on Friday and students boycotted classes for a third day running.

The Interior Ministry has dispatched police reinforcements to the Mediterranean city, which is a major tourist magnet, and warned that troublemakers would be swiftly dealt with.

"Throughout this week, as you well know, there have been violent incidents in Catalonia. They have been organised ... by groups who are a minority but are very organised," Interior Minister Fernando Grande-Marlaska told a news conference.

"Their actions, as we have already said, will also not go unpunished."

Some masked youths hurled stones at police late in the afternoon on one city street, but the vast majority of Friday's rallies were peaceful, with Barcelona's broad boulevards packed with people draped in the Catalan independence flag.

"We have always been peaceful people, but you get to a point where you get treated in such a way that people are getting angry," Carlota Llacuna, a 19-year old student from the Maresca region near Barcelona, told Reuters News Agency. "They put our leaders in prison."

One of the main ringleaders, Catalonia's former chief Carles Puigdemont, has so far escaped trial after he fled to Belgium in 2017 when the independence drive was thwarted.

Spain this week renewed its bid to get him extradited and he was briefly detained by Belgian police on Friday before a judge ordered his release pending a decision on the Spanish arrest warrant.

A court is meant to hear the case on October 29.

El Clasico postponed

In an apparent effort to hamper the protesters, a Spanish judge ordered on Friday the closure of web pages linked to a pro-independence group, Democratic Tsunami, which has been deftly directing its followers to various demonstrations.

However, as soon as its site was shuttered, the group migrated its homepage to a new url, sidestepping the ruling.

Democratic Tsunami is a new, secretive group that emerged in September and has drawn thousands of followers on both its website and social media.

Although it says it is committed to non-violent protests, many young demonstrators have battled police over the past three nights in Barcelona in scenes reminiscent of the some of the urban unrest that has rocked France over the past year.

Regional police said 16 people were arrested across Catalonia on Thursday, while health officials said 42 people needed medical attention.

Several main streets in Barcelona were closed to traffic because of Friday's marches, while regional trains and the city's metro were running on a reduced timetable.

Barcelona's main landmark, the multi-spired Sagrada Familia cathedral designed by Antoni Gaudi, was closed due to the protests, an official told Reuters.

The Spanish football federation (RFEF) said in a statement on that Barcelona's October 26 home match against Real Madrid, which is known as "el Clasico" and is one of the biggest rivalries in world sport, had been postponed due to security concerns.

Barcelona's El Prat airport cancelled 57 flights on Friday, airport operator Aena said.

Barcelona town hall said 700 rubbish containers had been set ablaze since protests began on Monday and estimated that the city had suffered damage totalling more than 1.5 million euros ($1.67m).