Some 5,000 people attended the sold-out concert for Spanish indie band Love of Lesbian, and were not required to observe social distancing. Attendees had to wear face masks throughout the concert in the Palau Sant Jordi arena.
The pilot event, which has been approved by health authorities, will serve as a test for whether similar events will be able to start up again.
Pre-concert testing was offered at three locations in the city and carried out by 80 nurses wearing full personal protective equipment. Some winced as nurses swabbed their noses.
By midday, of the 2,400 people already screened, three had tested positive and one had come into contact with a positive case, Dr Josep Maria Libre, one of the doctors overseeing the testing, told Reuters. They would not be able to attend the concert and would get a refund.
Attendees received the results of their antigen tests in 10 to 15 minutes via an app on their phones. The test and a mask were included in the ticket price.