A “small number” of Britons were injured in the attacks in Spain, the Foreign Office says, as the death toll rises to 14.
It also said it was “working to find out if any more need our help” and that the injured numbers could rise.
A van was driven into crowds on Las Ramblas in Barcelona on Thursday.
Shortly afterwards, five suspected terrorists were shot dead by police in the town of Cambrils in a second vehicle attack.
The Foreign Office said it had deployed extra staff in Spain.
The Foreign Office helpline for people calling from Spain is 112 and 012. From abroad it is 0034 93 214 21 24 and 0034 900 400 012.
It has also issued travel advice for those going to Spain.
“Our thoughts are with the victims of these terrible attacks and the people of Spain,” it added.
Prime Minister Theresa May condemned the “terrible attack”, adding the “UK stands with Spain against terror”.
The Spanish flag and the union jack are flying at half mast in Downing Street and at other government buildings.
The death toll has risen to 14 after a woman died in Cambrils, a popular seaside resort 110km (68 miles) south-west of Barcelona.
The other victims were all killed in Barcelona.
Spain’s civil protection agency said those killed and injured were from at least 34 different countries.
Spanish PM Mariano Rajoy has condemned what he called a “jihadist attack” and announced three days of national mourning.
A minute’s silence was held at noon local time on Friday.
Police are still hunting for the man who drove the van into people in Barcelona. Spanish media have named Moussa Oubakir, 18, as the suspect.
He is the brother of Driss Oubakir, whose documentation was allegedly used to rent the van involved in the attack.
Assistant Commissioner Mark Rowley, the UK’s head of Counter Terrorism Policing, said support had been offered to the Spanish authorities.
“Officers are at UK ports today ready to meet British holidaymakers returning to the UK from the region in order to take any witness accounts,” he added.
Anyone who has returned from Spain who has information about the attacks can also call the confidential hotline on 0800 789 321.