Carles Puigdemont also gave updated casualty figures during a brief news conference Thursday night.
Puigdemont says 12 people are confirmed dead and at least 80 have been hospitalized since the van jumped a sidewalk and swerved through a busy pedestrian area.
The region’s interior minister had tweeted earlier that 13 people had died in the attack.
Barcelona Mayor Ada Colau says a moment of silence will be held in the city’s main square at noon Friday “to show that we are not scared and we are more united that ever.”
Spain’s royal palace has condemned the deadly van attack in Barcelona, calling the perpetrators “assassins, simply criminals who will not terrorize us.”
A speeding van struck pedestrians in Barcelona’s Las Ramblas district, killing at least 13 people and injuring more than 50 others.
The royal palace’s statement, which was posted on Twitter, also said that “All of Spain is Barcelona. Las Ramblas will once again be for all.”
In a separate tweet, Prime Minister Mariano Rajoy said that “the terrorists will never destroy a united people who love liberty over barbarism. All of Spain is with the victims and families.”