Cardinal Joan Josep Omella welcomed families of the victims, as well as Spain's royals, the country's Prime Minister Mariano Rajoy and other top government officials, to the city's iconic Sagrada Familia basilica.
He read aloud a telegram of condolences sent by Pope Francis, who called the attacks a "cruel terrorist act" and a "grave offence to God."
The archbishop of Barcelona called for Spain to unite to work for a more peaceful world following the deadly dual vehicle attacks claimed by Islamic extremists, which left at least 14 people dead.
Opening hymn at the memorial service in Antoni Gaudi's iconic Sagrada Familia, the jewel of Barcelona. pic.twitter.com/kL6TslPCaz
— Eliza Mackintosh (@elizamackintosh) August 20, 2017
He said the presence of so many people was a "beautiful mosaic" of unity to work for a common objective of "peace, respect, fraternal coexistence and love".
The congregation heard prayers said for all those who'd been affected - and their families.
Meanwhile, authorities in Spain say the county's terror threat alert will remain at level four and security at tourist sites will be increased.
The search is continuing for the man who's believed to have driven a van into pedestrians in Barcelona on Thursday.
Australian Prime Minister Malcolm Turnbull says he still hopes a dual nationality British-Australian seven year old boy, who has been missing since the attack, can be returned to his family.
He said: "Our prayers go out to all the victims but we pray that little boy will be found and will be restored alive to his parents.
"It is a very tragic circumstance and one of great risk."
Two attacks on pedestrians Thursday and Friday in Barcelona and nearby Cambrils left 14 dead and over 120 wounded.
Spanish authorities have said the terror cell responsible for attacks in Barcelona and Cambrils has been dismantled, but a manhunt for one suspect is continuing.
At a press conference on Saturday Spanish interior minister Juan Ignacio Zoido said five members from the "terrorist cell from Barcelona" were shot dead, four were in custody and as many as two were killed in an explosion.
Later on Sunday, Barcelona FC will hold their first league game of the season, with 100,000 expected at the Camp Nou, where there will be a minute's silence.
