After two deadly vehicle attacks in Barcelona and a nearby resort, France's interior minister said his country reinforced police surveillance Friday of what is normally an open border between Spain and France.
The key suspect in Thursday's Barcelona attack, Moussa Oukabir, has been named as one of five men killed by police in the other attack site, Cambrils.
One Canadian was killed and four others were hurt when a van plowed into crowds of tourists on a popular street in Barcelona, Prime Minister Justin Trudeau confirmed Friday.
Suspected terrorists have been behind the previous attacks.
"I don't want to overstate my role, but seeing the terror on people's faces was just frightful".
"So I think we should all in our private moments say a prayer for that little boy".
German Chancellor Angela Merkel, speaking after media reports that some Germans were among those killed, said Islamist terrorism "can never defeat us" and vowed to press ahead with campaigning for a general election in Germany in September. Aged from 21 to 34, they had no history of terror-related activities.
Police have arrested four people in connection with the attacks - three Moroccans and a citizen of Spain's North African enclave of Melilla.
Bruno Gulotta, 35, was on vacation in Barcelona with his partner Martina and two children, 5-year-old Alessandro and daughter Aria, who is a few months old.
Tony Cadman, whose Facebook profile says he lives in Sydney and is from Gillingham, Dorset, posted a photograph of Julian, writing: "My grandson, Julian Alessandro Cadman is missing".
"The Foreign Office is working to establish if any British nationals were involved in this appalling incident and we are in close contact with the authorities in Spain, who have our full support".
We want to hear from you.
Four others have been arrested, one in Alcanar after Wednesday's explosion and three in the Catalan town of Ripoll after the Barcelona attack. Police destroyed explosive belts the men had been wearing, though they turned out to be fake.
"It is clear that the intention of those who carried out this appalling, evil act was to kill, wound and terrorise innocent people enjoying the freedoms we share and cherish", he said.
She said the residents of Barcelona remained "defiant" in the face of the violence, and that thousands of mourners gathered in the city's main square on Friday to observe a minute of silence and march through the city's streets.
Three days of mourning have been declared by the government of Catalonia.
"They were preparing one or several attacks in Barcelona and an explosion in Alcanar stopped this as they no longer had the material they needed to commit attacks of an even bigger scope", said Josep Lluis Trapero of Catalonia's police.
He is suspected of using his brother's documents to rent the van that mowed down people on the famous boulevard.
He was among five people shot dead by police in the resort town of Cambrils in the early hours of Friday, after they crashed their vehicle into a group of people, killing one woman.
Oukabir, 17, was the younger brother of one of the men arrested on Thursday.
Witnesses said the white van zigzagged at high speed down Las Ramblas, a busy avenue thronged with tourists, knocking down pedestrians and leaving bodies strewn across the ground.