Tourists have joined Spanish royalty and the nation's prime minister in mourning victims of two deadly vehicle attacks at a memorial service held at Barcelona's famous Sagrada Familia Basilica.

Underneath the soaring arches mourners gathered for mass and to remember the victims of the terror attacks that killed 14 people.

The terrorist cell was reportedly planning a larger assault on the landmark building where the service was held.

But an accidental explosion, likely from a botched homemade bomb, forced them into a more hurried attack.

Spanish Interior Minister Juan Ignacio Zoido says the terror cell behind the attacks has been "broken" but police are still searching for the driver responsible for the Barcelona attack.