People took photos and listened to audio guides after Archbishop of Barcelona Joan Josep Omella led representatives of healthcare workers into the church.
The world famous building - which was designed by architect Antoni Gaudi and started in 1882 but is still unfinished - closed almost four months ago. But for the next two weekends it will be open to essential workers, including those in healthcare, the police and NGOs, who will be able to explore without the usual crowds.
The goal is to recognise and pay tribute to Barcelona residents, "especially those who have been on the front lines fighting and working to prevent Covid-19", according to a statement on the basilica's website.
A second phase of reopening will see the lofty and famously unfinished church welcome Barcelona's residents for free, while a third will allow domestic and international tourists to visit.
The basilica was ranked the sixth most visited tourist attraction in the world based on bookings, according to TripAdvisor's 2019 list, and the most visited in Spain.
Spain has registered 205,545 coronavirus cases and 28,385 deaths, according to health ministry data, making it one of the worst affected countries in Europe.