BARCELONA, Spain — At least 51 people have been arrested in a massive crackdown on drug dens in and around Spain's northeastern city of Barcelona, police said Wednesday.

The latest anti-drug operation involved more than 1,000 agents conducting more than 40 searches in flats in the city center neighborhood of Raval and other nearby locations, the Mossos d'Esquadra regional police force said.

National and local police agents also took part in the raids, assisted by low-flying helicopters.

Flats used for storing drugs and as "narcoflats" where users look to get their heroin and cocaine fixes have been spreading in the poorest areas of Spain's biggest cities in recent years.

Similar amounts of people were arrested in the two previous crackdowns, in June 2019 and October 2018.