The vehicular attacks that fatally crushed at least 14 people in Spain may have been hatched in a house that the plotters were using as a bomb factory, police said Friday.

At least four suspects were arrested in connection with what Spanish authorities say appears to be a sophisticated and far-reaching plot that could have been much worse.

The attacks carried out on Thursday in Barcelona and hours later in the seaside resort of Cambrils wounded scores, shattering Spain's blissful summer holiday season. The Islamic State claimed responsibility for the Barcelona assault, which accounted for most of the victims.

It was the worst terrorist attack to hit Spain in 13 years and echoed similar assaults by Islamic extremists in France, Germany, Britain and elsewhere, where vans and trucks have been used as instruments for killing.

Police investigators said they were working under the assumption that the attack in Barcelona, where a van careened down a crowded pedestrian boulevard, and a second incident in which five men in a car hit people in Cambrils were related.

They also linked both assaults to an explosion the night before in the town of Alcanar, 120 miles southwest of Barcelona, that they now believe was evidence of a factory the plotters had been using to make a truck bomb.

Had they succeeded, the carnage on Thursday could have been far more devastating.

Authorities had been warning of an impending attack for some time, having raised the terrorism threat alert to its second-highest level in 2015. Since the beginning of this year, Spain has arrested 54 people suspected of links to Islamic terrorism, according to the Interior Ministry.

A spokeswoman for Catalan police identified Moussa Oukabir, 17, as a suspect and possibly the driver of the van that careened down Las Ramblas, a major Barcelona street crowded with tourists, killing 14. The driver fled the scene on foot and is now the focus of a sweeping manhunt.

One of Oukabir's older brothers was among four people arrested in connection with the attack.

Another police official said Oukabir may have been among five assailants killed in the attack in Cambrils, 60 miles southwest of Barcelona, when Spanish police opened fire on a vehicle after it plowed into a crowd, killing one woman.

Investigators said they were working under the assumption that both attacks stemmed from an explosion late Wednesday at a residence in Alcanar that they had initially discounted as a gas accident.

A European counterterrorism expert, who was briefed on the details of the investigation, said police believed the assailants had been manufacturing a sizable device that they aimed to pack into a large truck.

Friday morning, Las Ramblas in Barcelona was thronged again, almost as if nothing had happened. But the mood was subdued, with few customers in the shops.

At the spot where the van driven by the assailant had halted, and where many people were killed or injured, was a pavement mosaic by Joan Miró, the city's most famous modern artist. In the centre of the mosaic lay a makeshift memorial to the victims that included flowers, candles and notes, with one reading, "Barcelona weeps but does not surrender".

The victims came from at least 34 countries, Spanish authorities said, highlighting how the assailants chose to target one of Europe's busiest tourist centres at the height of the summer season. Those killed included a 74-year-old Portuguese woman walking with her granddaughter, two Italians and an American. A German official said that several citizens were "fighting for their lives" in the hospital.

One Australian is missing and two women from NSW are in a serious but stable condition.

Family and friends of a seven-year-old Australian boy missing after the Barcelona terror attack are pleading with people worldwide on social media to help find him.

Julian Cadman became separated from his seriously injured mother Jom in the terrorist attack on Friday. The mother and son from Sydney were in Spain for a wedding this weekend.

Just before a moment of silence for the victims at noon Friday, church bells rang out and people began to move toward the Plaza de Cataluña, the central square of Barcelona. At another makeshift shrine there, two women, one wearing a hijab, were weeping and holding each other. People nearby chanted in the Catalan language, "The people together will not be beaten," and, "We are not afraid".

Political tensions erupted into the open when Catalans remonstrated loudly, with a man with a Spanish flag wrapped around his legs shouting, "This is not the place!" Moments later, opposing groups chanted at each other in Spanish and Catalan.

Others were more reflective, consumed with sadness and worried that the plague of vehicular attacks across Europe over the past two years had now reached Spain.

"I am anxious, nervous, my chest is tight, but at the same time people are going out," said Estella Gil, a teacher's assistant, who had come to Las Ramblas in a show of solidarity. "Usually, I walk here feeling safe. Now, I am afraid, really afraid."

Moussa Oukabir's brother Driss Oukabir was arrested Thursday after going to police and claiming that his identity documents had been stolen. A national police official, speaking on the condition of anonymity to discuss a continuing investigation, said at least three vans had been rented under Driss Oukabir's name. He has denied any connection to the attacks.

Two years ago, a person using the name Moussa Oukabir wrote in an online forum that if he were king, he would "kill all infidels and only spare Muslims who follow the religion".

Three of the arrested men were of Moroccan origin, while the fourth comes from Melilla, a Spanish enclave in North Africa adjoining Morocco. One suspect detained in Ripoll, about 65 miles north of Barcelona, has not been identified. It was not clear if his arrest was in connection with the attack in Barcelona, the second assault in Cambrils, or both.

As the scope of the attacks began to emerge, perhaps the most troubling aspect was the apparent existence of a terrorist cell that coordinated the assailants' actions which might, but for an accident while mixing chemicals for explosives, have been far more deadly.

That blast in Alcanar, which killed one person and injured several others, occurred just before midnight Wednesday and was first reported as a gas explosion. But as attacks unfolded in Barcelona and Cambrils later Thursday, police soon made the connection to Alcanar.

The attack in Cambrils was halted by police only after the driver of a compact Audi A3 rammed his vehicle into a group of pedestrians. Seven people were wounded, and one woman later died of her injuries.

The Spanish newspaper La Vanguardia reported that the Audi had run through a security checkpoint at the entrance to the town, prompting a police chase. After driving into pedestrians, the five occupants emerged, wielding knives and an ax. But police quickly descended, killing all five before they could commit any further mayhem.

The five assailants appeared to be wearing explosive vests, although Catalan police said Friday that the explosives were fake. The Islamic State, also known as ISIS and ISIL, claimed responsibility for the attack in Barcelona, but there has been no such claim as yet for the events in Cambrils.