BARCELONA, Spain >> Violent clashes erupted for a second night in Barcelona between police and protesters angry about the conviction of a dozen Catalan separatist leaders, as Spain launched an investigation today into an activist group organizing the demonstrations.

Thousands of people held vigils near the Spanish government offices in Catalonia’s four provinces. But the gatherings turned into melees, with protesters hurling firecrackers and other objects at anti-riot officers and kicking temporary fences around the official buildings.

In Barcelona, protesters sang the Catalan anthem and shouted “The streets will always be ours” and “Independence,” while they called Spanish police “occupying forces” and urged them to leave Catalonia. Demonstrators erected improvised barricades with trash bins, fences and piles of cardboard that they set on fire.

The greatly outnumbered riot police went in with shields and batons striking protesters in their path. They hit people on the legs mostly and fired foam-type anti-riot bullets that dispersed the crowds.

There were also clashes in Girona, Lleida and Tarragona as well as smaller towns across Catalonia.

Most impromptu protests were responding to an online campaign by Tsunami Democratic, a loose, leaderless grassroots group that uses encrypted messaging apps to call for peaceful disobedience.

Spain’s caretaker interior minister, Fernando Grande-Marlaska, said authorities were investigating the group.

Protests were initially called by ANC and Omnium, two grassroots pro-secession groups whose leaders Jordi Sánchez and Jordi Cuixart were sentenced Monday to nine years in prison for sedition.

In written answers to questions sent to Sánchez in prison, the activist-turned-politician defended the non-violent nature of the Catalan independence movement.

“The risk of frustration is always there among certain people when there is a movement so big of millions,” Sánchez wrote. “But I don’t see any future for violence. We don’t practice it, nor do we defend it.”

Seven politicians were also given prison terms of around a decade in Monday’s Supreme Court ruling and three more were fined for disobeying the laws as they pushed ahead with a banned referendum on Oct. 1, 2017, and briefly declaring independence based on its results before Spain crushed the defiance.

Today, protesters also blocked highways, smaller roads and railway tracks for brief periods following an attempt to besiege Barcelona’s international airport Monday night that left thousands of travelers stranded. Many, desperate to catch their flights, were forced to walk with their luggage on highways and across fields.

The airport authority said 110 flights were canceled Monday and 45 today, when nearly 1,000 operated normally.

Officials said six people had been arrested — three on Monday and three today. More than 170 people were reported injured, including about 40 police officers and a protester who lost an eye.

The court’s ruling Monday also barred the 12 convicted Catalan separatists from holding public office. That has an immediate impact in the upcoming Nov. 10 Spanish election because six of them were planning to run as candidates to the national parliament.

The verdict is likely to be a central issue in the election campaign but “it is unlikely to substantially alter the electoral outlook unless the situation worsens significantly in the region,” said Antonio Barroso, a political risk analyst with the London-based Teneo consulting firm.

He said Catalan separatist politicians wanted to use the backlash against the ruling to woo pro-independence voters to the polls.

Others have feared that swelling support for Catalan separatism because of the sentences could make the next political term even more key to either breaking the deadlock with separatists or making it a chronicle problem.

Spain’s caretaker prime minister and Socialist leader, Pedro Sánchez, who won the April election but failed to get support for a minority government, is hoping to remain in office.

Even from the early hours after the Supreme Court ruling was issued, very different views emerged from Madrid and Catalonia. While Sánchez called for beginning a “new phase” and urged Catalan separatists to abide by the law, the ruling invigorated the wealthy region’s independence movement, with many of its leaders making new calls to work toward effective secession or repeating the slogan “we will do it again.”

Cuixart told the Associated Press by email via his lawyer that he and the others sentenced by the Supreme Court will take their case to the European Court of Human Rights.

“We have the moral obligation to denounce (Spain’s) authoritarian drift and appeal to European citizens to defend human rights,” Cuixart wrote Tuesday.

The Catalan regional president, Quim Torra, said secessionists won’t give up their fight.

“A referendum is the most positive solution for solving this situation,” he told foreign reporters in Barcelona on Tuesday.

The caretaker Spanish foreign minister, Josep Borrell, soon due to become the European Union’s top diplomat, said the court’s ruling would not resolve the underlying political problems. Only dialogue “in the framework of the constitution” could do that, he said. Spain’s constitution says the country is indivisible.

“Yesterday, today and tomorrow it is and remains a political problem that has to be solved,” Borrell told foreign reporters, adding that Catalan separatists shouldn’t ignore Catalans like him who are against independence.

Protests are likely to extend throughout the week. A three-day student strike begins Wednesday in the region, and a worker’s union has called a separate strike for Friday.