MADRID (AP) — Spanish police arrested at least 55 people Monday in Barcelona during a massive crackdown on the squalid apartments used for migrant and drug trafficking in the northeastern Catalonia region.

More than 700 investigating agents and 150 local police officers took part in Monday's anti-drug trafficking operation, the regional Mossos d'Esquadra police said in a series of tweets.

An Associated Press photographer saw scores of heavily armed police bringing suspects out of a building and hauling them into police vans. Police helicopters flew over Barcelona for most of the morning.

Police said the operation came after a two-year investigation into the rings running the drug den apartments.

A court in Barcelona authorized 40 searches on such apartments, also known as "narco flats," in the central Raval and Gotic neighborhoods, according to the regional judiciary administration.

With crime statistics on the rise in recent years, Barcelona residents have blamed the growing insecurity on the spread of illegally occupied flats used as drug distribution and consumption centers, mostly for heroin.

The Catalan police said before Monday's crackdown, authorities had dismantled 100 such apartments and arrested 139 people linked to drug trafficking in the past two years.