Spain's King and Queen have joined mourners a mass to remember the 14 people killed in the terror attacks in Barcelona and a nearby coastal town.

The service came as the hunt for the main attack's suspected van driver continued as police said they had linked three rental vans to the fugitive.

Investigators also said the terror cell had stored more than 100 gas tanks at a house they accidentally blew up on the eve of the twin attacks.

There was tight security in Barcelona with armed police stationed outside the iconic Sagrada Familia Basilica cathedral along with snipers on rooftops.

Spanish Prime Minister Mariano Rajoy was also among the politicians and dignitaries who had gathered to honour those killed and injured in last week's attacks.

Hundreds were present for the mass inside the city's iconic cathedral. Credit: PA

Cardinal Joan Josep Omella, who led the mass, said the presence of so many people in the Gaudi-designed cathedral was a "beautiful mosaic" of unity to work for a common objective of "peace, respect, fraternal coexistence and love."

The cardinal, who is also the archbishop of Barcelona, also read out a message of condolence sent by Pope Francis, who called the attacks a "cruel terrorist act" and a "grave offense to God."

Mourners gathered for the mass in Barcelona on Sunday. Credit: AP

Around 34 nationalities were among those wounded on the famous tourist promenade of Las Ramblas and in the coastal town of Cambrils, which lies around 70 miles to the south west.

Authorities say they have identified some of the victims as Italian, Portuguese, Spanish, Spanish-Argentine and American citizens.

Armed police guarded outside the cathedral during the service. Credit: PA

Younes Abouyaaqoub, a 22-year-old Moroccan, is suspected of driving the van used in the attack on Las Ramblas.

He has remained at large since Thursday's atrocity, which left at least 120 injured.

Police are hunting Younes Abouyaaqoub is the suspected driver in the van attack.

Police have linked three rental vans to the fugitive which were reportedly rented using his credit card.

An official said one of the vans was used in the Barcelona attack, while another was found in Vic, 44 miles north of the city.

It was found on the road to Ripoll, where all the main attack suspects lived, and the third was found in Ripoll itself.

A top Spanish police official said the cell of 12 people "had planned one or more attacks with explosives" in Barcelona and had also used a car and motorcycle in the attack planning.

A woman looks at candles and flowers laid in tribute to the victims. Credit: AP

Josep Lluis Trapero said more than 100 butane gas tanks were found at the home in Alcanar that was destroyed Wednesday.

Investigators also found ingredients of the explosive TATP that has been used by Islamic State extremists in other attacks.

Mr Trapero said the search for Abouyaaquoub has been complicated because authorities haven't been able to positively identify the human remains at the house.

A 14th victim who died in a second van attack in the Catalan seaside resort of Cambrils has been identified as Spanish woman.

Five men were shot dead by police as they launched the attack which also injured six people in the early hours of Friday.

Police at the scene where five suspects were shot and killed in Cambrils. Credit: PA

On Friday it emerged that one of the men, Moussa Oukabir, is thought to have rented the van used in the Barcelona attack.

The teenager, believed to have been 17 or 18-years old, is suspected of using his brother's documents to hire the vehicle.

He reportedly died along with Said Aallaa, 19, and Mohamed Hychami, 24,

Suspects Moussa Oukabir, Mohamed Hychami and Said Aallaa are believed to have been killed.

The identities of the other two dead suspected jihadists are yet to be confirmed by police.

Four men, aged 21, 27, 28 and 34, who were arrested in connection with the attack remain in custody. Three are Moroccan and one Spanish.

Police said none of them were previously known to the security services for terror-related reasons.

Moussa Oukabir's older brother, Driss Oukabir, is reported to be one of those detained.