"Demonstrations have taken place in Barcelona and other areas of the Catalonia region and may continue for some days", states the Foreign and Commonwealth Office website, according to Britain's daily Express.
The bloc urged both sides to avoid confrontation in favour of dialogue.
The central government and national courts branded the referendum illegal.
The vote Sunday saw massive violence as Spanish police attempted to prevent the vote.
Some police officers were seen firing rubber bullets, storming into polling stations and pulling women by their hair. There were no reports of trouble.
Spain's Banco Sabadell, the country's fifth-largest bank with a large business in Catalonia, also attempted to calm jitters.
Spain's Caixabank, Catalonia's largest lender, and ministers reassured bank customers that their deposits are secure from the growing crisis. Puigdemont is seeking international mediation following the vote, saying he wants to reach an understanding with the Spanish government, and avoid a "traumatic break".
The decision came amid violent local clashes as police attempted to shut down an unofficial Catalan independence referendum.
The Catalan government now says it plans to unilaterally declare independence from Spain within 48 hours.
But after Sunday's violence, his address risked further fanning resentment in Catalonia.
The authorities claim that the demonstrations obstructed police operation. The Spanish response to the referendum and how other nations react to it is also in focus.
"If the law does not give you what you want, you can oppose the law, you can work to change the law, but you cannot ignore the law", Timmermans said. Public opinion studies have repeatedly shown that most Catalonia residents support a referendum. However, the studies found they are about evenly divided on the issue of independence from Spain.
He is a 16-time Grand Slam champion, winning the 2017 US Open.
The mood was not jubilation.
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