A spokesman with the Mossos d'Esquadra regional police said the church has been evacuated and several streets cordoned off to allow police work in the area.
The spokesman declined to be identified by name, following the force's protocol.
Barcelona, a tourism hub in northeast Spain's Catalonia region, and a nearby coastal town were targets of extremist attacks that killed 16 people last month. The Islamic State claimed the attacks.
Spain has kept its anti-terrorism security warning one level below the maximum since mid-2015.