Several thousand demonstrators gathered outside the Barcelona headquarters of Spain’s national police force on Tuesday amid strikes in protest at police violence during the disputed referendum on Catalonian independence.

As Madrid comes under growing international pressure to resolve its worst political crisis in decades, the crowd gathered in the Catalan capital, chanting “independence”, calling Spanish police an “occupying force” and urging them to leave the region.

In Barcelona, municipal police said about 15,000 people had stopped traffic as they rallied, many draped in the blue, yellow and red Estelada flag used by Catalan separatists, shouting: “The streets will always be ours.”

The protest came as several small labour unions and grassroots pro-independence groups urged workers throughout Catalonia to go on partial or full-day strikes. 

“An attack on democracy without precedent in recent times calls for a united response,” said Javier Pacheco, the secretary general in Catalonia of the Comisiones Obreras (CCOO) union. “We have called on all sectors to take part.”

However, while the Catalan sections of the CCOO and UGT unions are supporting the strike, the two organisations’ national leaderships are not.

Schools and universities were shut on Tuesday and most small businesses were closed after unions called for the stoppage to “vigorously condemn” the police response to the poll, in which Catalonia’s leader said 90% of voters had backed independence from Spain.

Metro stations in Barcelona that are usually busy were deserted as services were cut back sharply, and the Boqueria market was almost empty. Elsewhere, the response to the strike call was patchy. There were no reports of disruptions affecting big industry or Barcelona’s airport.

“People are angry, very angry,” said Josep Llavina, 53, a self-employed worker who had travelled to Barcelona from a nearby town to participate in the protest outside the police building. “They brought violence with them. They have beaten people who were holding their hands up. How can we not be outraged?”

José Bolivar, 54, a town hall employee, said: “I disagree with the strike. In fact, at work nobody told me anything about a strike. So I decided to come.”

Office worker Antonia Cuello, 37, was in two minds about the industrial action. “On one side it is a hassle to try to get to work in the midst of a strike,” she said. “We are suffering this because a few decided to behave in an improper way. On the other hand, I understand the circumstances surrounding the strike.”

Barcelona’s contemporary art museum and the Sagrada Família, the basilica designed by Antoni Gaudí and one of the city’s most popular tourist sites, were expected to join the strike.

FC Barcelona said it would take part in the strike, adding that it would close its headquarters and that none of its professional or youth teams would train.

The central government has vowed to stop the wealthy north-eastern region, which accounts for a fifth of Spain’s GDP, breaking away from Spain and has dismissed Sunday’s poll as unconstitutional and a “farce”.

At least 893 people and 33 police officers were reported to have been hurt on Sunday after riot police stormed polling stations, dragging out voters and firing rubber bullets into crowds.

Violent scenes played out in towns and cities across the region as riot police moved in to stop people from casting their ballots.

The UN rights chief, Zeid Ra’ad al-Hussein, said he was “very disturbed” by the unrest while the European Council president, Donald Tusk, urged Madrid to avoid further use of violence.

The European parliament will hold a special debate on Wednesday on the issue.

“We call on all relevant players to now move very swiftly from confrontation to dialogue. Violence can never be an instrument in politics,” said the European commission spokesman Margaritis Schinas, breaking weeks of virtual EU silence on the Catalan issue.

The government of the Spanish prime minister, Mariano Rajoy, held emergency talks after the Catalan leader, Carles Puigdemont, declared on Sunday that Catalonia had “won the right to an independent state”.

Puigdemont has appealed for international mediation to help solve the crisis and called for police deployed to Catalonia from other parts of Spain to be removed.

The regional government said 2.26 million people had taken part in the poll, or just over 42% of the electorate.

But any attempt to unilaterally declare independence is likely to be opposed not just by Madrid but also a large section of the Catalan population, a region of 7.5 million people that is deeply split on the issue.

Puigdemont said he would present the results to the region’s parliament, where separatist lawmakers hold a majority, and which has the power to adopt a motion of independence.