Thirteen people were killed and dozens injured after a van hit crashed into pedestrians in the popular tourist area of Las Ramblas, Barcelona.
Reports say two gunmen then stormed into a restaurant in the city.
Spanish police said a “massive crash” involving a white van has taken place an primer minister Mariano Rajoy described it a “jihadist attack” carried out by so-called Islamic State.
Reports say a second attack has been foiled and five suspects were killed by officers.
British tourists have been warned by the Foreign and Commonwealth Office to avoid the area around Placa Catalunya.
The Foreign and Commonwealth Office has updated its travel advice to say: “The British Embassy in Madrid and Consulate General in Barcelona are in contact with local authorities and urgently seeking further information following reports of an incident in central Barcelona; local authorities have advised people to stay inside and stay away from the Las Ramblas area of the city; if you’re in the immediate area you should take care and follow the advice of the local security authorities; press reports suggest that some public transport, particularly the metro, has been affected; this page will be updated as the situation develops.”
Spanish authorities have set up a helpline for people affected by the incidents: 0034 93 214 21 24.
Besides being a popular city break destination, Barcelona is a major cruise port, attracting thousands of passengers every week during the summer peak.
EasyJet said: “EasyJet’s thoughts are with the family and friends of the victims of the incident in Barcelona.
“The safety and security of its passengers and crews is always easyJet’s highest priority.
“Our schedule is operating as normal, however we would advise that all passengers due to fly with us to or from Barcelona today check the status of their flight on our Flight Tracker page.
“EasyJet will continue to comply with all recommendations and guidance from the Spanish authorities.”
Abta also warned people due to travel to Barcelona. In a statement, it added:
“Holidaymakers in Barcelona should stay away from the Las Ramblas area of the city, follow the advice of local authorities and monitor the Foreign & Commonwealth Office (FCO) travel advice for Spain. This advice also applies to people imminently due to travel to Barcelona.”