Protesters have clashed during a pro-Spanish unity demonstration in Barcelona, throwing chairs at each other and leaving one police officer injured as authorities stepped in to break up the fighting.

Some 65,000 people are thought to have gathered in the Catalan capital to mark Spain’s national day and call for the country’s unity against attempts at secession.

The turnout was the highest since the police started keeping track of the figures three years ago, a spokesman from the city’s urban guard said.

Two small groups of protesters clashed in the city centre, throwing chairs near the Plaza de Catalunya.

It was unclear what sparked the violence and Barcelona’s municipal authorities said the groups’ ideological leanings had not been established.

Before the fighting began, around 300 people began chanting at the local Mossos d’Esquadra police force and some called them “traitors”, according to the El Diario newspaper.

The officers backed away to avoid a confrontation, the site said, minutes before the brawl broke out some distance away.

Footage of the fight showed chairs flying in both directions, before Barcelona’s urban guard and regional police removed a few dozen protesters on each side. One officer suffered minor injuries.

The brawl reportedly did not disrupt the nearby unionist demonstration.

A separate protest of around 200 supporters of far-right and Spanish nationalist groups ended on Barcelona’s Montjuic hill with speeches and the burning of a “Senyera,” the unofficial flag that has become a symbol for Catalan separatists.

Catalonia referendum protests – in pictures Show all 10 1 /10 Catalonia referendum protests – in pictures Catalonia referendum protests – in pictures Demonstrators block a Guardia Civil vehicle as they try to leave the Department of External Affairs, Institutional Relations and Transparency of the Catalan Government office in Barcelona AP/Emilio Morenatti Catalonia referendum protests – in pictures Demonstrators react as they try to stop the car carrying Xavier Puig, a senior at the Department of External Affairs, Institutional Relations and Transparency of the Catalan Government office, after he was arrested by Guardia Civil officers in Barcelona AP/Emilio Morenatti Catalonia referendum protests – in pictures A demonstrator reacts as he tries with others to stop the car carrying Xavier Puig, a senior at the Department of External Affairs, Institutional Relations and Transparency of the Catalan Government office, after he was arrested by Guardia Civil officers in Barcelona AP/Emilio Morenatti Catalonia referendum protests – in pictures Spokeswoman of the Catalan pro-independence anticapitalist party "Candidatura d'Unitat Popular - CUP" (Popular Unity Candidacy), Ana Gabriel, talks to the media in Barcelona Josep Lago/AFP Catalonia referendum protests – in pictures Republican Left of Catalonia party's (ERC) Member of Parliament Joan Tarda (C) attends a demonstration outside the regional Economy Ministry in Catalonia during a police search for documents connected with the organisation of the Catalan independence referendum, in Barcelona EPA/Alejandro Garcia Catalonia referendum protests – in pictures A man holds pro-referendum poster next to a Spanish Civil Guard who stands in front of the Economy headquarters of Catalonia's regional government in Barcelona. The operation comes amid mounting tensions as Catalan leaders press ahead with preparations for an independence referendum on October 1 despite Madrid's ban and a court ruling deeming it illegal Josep Lago/AFP Catalonia referendum protests – in pictures People hold placards reading "Democracy" as they protest in front of the Economy headquarters of Catalonia's regional government in Barcelona AFP Catalonia referendum protests – in pictures A crowd of protesters gather outside the Catalan region's economy ministry after junior economy minister Josep Maria Jove was arrested by Spanish police during a raid on several government offices, in Barcelona Reuters/Albert Gea Catalonia referendum protests – in pictures People holding 'Esteladas' (Catalan pro-independence flags) attend a protest near the Economy headquarters of Catalonia's regional government Lluis Gene/AFP Catalonia referendum protests – in pictures People demonstrate on a Spanish Civil Guard Police car outside the Catalan Vice-President and Economy office as police officers holds a searching operation inside David Ramos/Getty Images

The scenes came as Catalan leader Carles Puigdemont engaged in back-and-forth brinkmanship with Spanish Prime Minister Mariano Rajoy over his delayed declaration of regional independence.

Mr Puigdemont reacted to the Spanish ultimatum to backtrack on his independence plans or face a suspension of regional powers, tweeting: “We demand dialogue and the response is to put article 155 on the table. Message understood.”

He had earlier proposed to regional lawmakers to freeze the implementation of secession to allow time for negotiating independence with Spain. Mr Rajoy has rejected any dialogue as long as secession is on the table.

The only other acknowledgement of Madrid’s warning came from Mr Puigdemont’s deputy, Catalan vice president Oriol Junqueras, who tweeted to Mr Rajoy: “A sincere dialogue is what the international community wants and what Catalonia expects, not confrontation and new threats.”