Specialist bomb disposal experts have been sent to the area to check a van, police said.
Several roads have also been closed.
At about 8.30pm local time, the local police force, the Mossos d'Esquadra, said it was examining reports of a suspicious vehicle in the area as part of an anti-terrorist operation. The worst of these happened in the city's main street of La Rambla, where a rented van mowed down pedestrians, killing 15 and injuring more than 130 others.
Eyewitnesses reported armed police looking through bins, as they told shopkeepers to close their businesses.
The Sagrada Familia, one of Barcelona's most popular tourist attractions, was the intended target of the terrorists who carried out last month's atrocity in the city's famous Ramblas and the seaside resort of Cambrils.
The warped jihadis planned to pack three vans full of gas cylinders and explode one van on Las Ramblas, a second by the Sagrada Familia, and a third in the Barcelona's port area, according to Spanish news site El Espanol.
Police said earlier they had evacuated the site as part of an anti-terrorism operation.
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