Spanish police on Wednesday said they had seized around four million counterfeit medical-grade face masks at a hotel during a raid on a gathering that violated virus restrictions.

Officers found the masks inside cardboard boxes on the ground floor and in the warehouse of the hotel in Leganes on Madrid's southern outskirts, police said in a statement.

The boxes were "ready to be transported" and marked as containing high-protection KN95 and FFP2 face masks, it said. 

In addition, police found a printer that was capable of printing masks with the so-called CE mark, reflecting conformity to European standards.

"The material which was seized leads investigators to believe that non-approved masks were being put through a (CE-mark) printing process so they could be put on the market as certified masks," the statement said.

Police searched the hotel after being notified of a gathering of 48 youths that violated restrictions regarding the number of people allowed to meet up.

Officers detained the hotel manager on suspicion of falsification of documents.

Spain has been hard-hit by the pandemic, recording nearly 66,000 deaths from just over three million cases so far.