The pre-concert testing took place at three locations in Barcelona, carried out by 80 nurses wearing full personal protective equipment.
By midday, three out of 2,400 people already screened had tested positive and one had come into contact with a positive case, said Dr Josep Maria Libre, a doctor who oversaw the testing.
Those who tested positive were unable to attend the concert but would be entitled to a refund.
Attendees received their antigen test results in 10 to 15 minutes via an app on their phones. The test and a mask were included in the ticket price of between €23 and €28 (£20 and £24).