Fresh protests broke out in the streets of Barcelona on Thursday, with up to 25,000 high school and university students calling for a peaceful solution to Spain's biggest political crisis in 40 years.

The city of Barcelona is also preparing for a general strike on Friday, while the Spanish authorities are preparing to send in reinforcements over the weekend to quell the recent violence.

Wednesday night saw the worst night of violence yet, with protesters setting cars and trash cans on fire, setting up barricades in the streets and throwing acid at police officers.

Spain's interior ministry said 96 people have been arrested in the region since Monday, four of whom are in custody. Fifty-nine people have been injured and protesters expressed anger that Spanish police had resorted to tear gas.

The protests broke out on Monday, hours after the Spanish Supreme Court sentenced nine Catalan politicians and activists to between nine and 13 years behind bars, following Catalonia's 2017 failed independence referendum.

The court found them guilty of sedition, disobedience and misuse of public funds. 

Among those jailed was the former Catalan vice president, Olio Junqueras, who received a 13-year prison sentence. 

At protests on Thursday, high school and university students called for a stop to the violence, stressing that dialogue still had not worked and their demands had fallen on deaf ears.

Barcelona's El Prat airport was temporarily shut down on Monday with protesters hoping to bring operations to a standstill. The chaos has since spread to other parts of Catalonia including Lleida and Tarragona. 

This most recent wave of violent protests has been fueled by a clandestine group called Tsunami Democratic, leveraging social media and encrypted messaging to communicate the direction of the protest. However, there is widespread sentiment in the region for independence.

On Wednesday night, Spain's prime minister, Pedro Sanchez, called on the region's president, Quim Torra, to condemn the violence. 

For the first time, Torra commented on the situation – calling for protesters to march peacefully, but has yet to condemn the violence explicitly.

Addressing the public on Monday outside the Catalan government headquarters, Torra referred to the sentencing as "an act of vengeance, not justice." 

The Catalan president added the verdict was "anti-democratic" and goes against "the right to self-determination and independence." 

On Friday, two trade unions called for a general strike across the region, which will cause major disruption to trains and other public services. Protesters will march from across the region, aiming to meet in Barcelona at approximately 15:00 local time. Protests are expected to continue this evening ahead of Friday's general strike.