Spanish officials say at least 13 people died and more than 80 were injured when a van plowed into pedestrians in Barcelona's famous Las Ramblas tourist area on August 17.
The president of the Catalonia region, Carles Puigdemont, said police arrested two people over what they called a “terrorist attack."
The extremist group Islamic State (IS) claimed responsibility, the IS-affiliated Amaq news agency said.
Spanish Prime Minister Mariano Rajoy said he was heading to Barcelona to coordinate efforts to reinforce security.
He said “the terrorists will never destroy a united people who love liberty over barbarism.”
"They are assassins, criminals who won't terrorize us,” the royal palace said.
U.S. President Donald Trump took to Twitter to condemn the attack and say that the United States "will do whatever is necessary to help." "Be tough & strong, we love you!" he added.
French President Emmanuel Macron said, "We remain united and determined."
Russian President Vladimir Putin called for the world to unite in an "uncompromising battle against the forces of terror."
In the past year, vehicles have been used to ram into crowds in a series of militant attacks across Europe, killing more than 100 people in Nice, Berlin, London, and Stockholm.
Based on reporting by AFP, Reuters, AP, and El Pais