Shoppers in Brazil found they had unwittingly bought cocaine when they opened soap powder they had purchased at a supermarket in Sao Paulo.

At least 80kg (176lb) of the drug was found in the store in the Ermelino Matarazzo district, police said.

Tweeting a picture of the cocaine packages, police said they were "cleaning the streets of criminals".

Detectives believe the shop, where the soap powder was bought on Monday, is involved in drug trafficking.

One theory, police say, is that the drug-stuffed soap powder boxes were put on the shelves by mistake.

One customer tried to return his soapbox to the store while another handed his in at his local police station, the G1 news website reported.

In a press release, Sao Paulo military police said "individuals were unloading more of the same product" when officers arrived at the shop, adding that the "criminals tried to escape".

Four people have been arrested, including three employees of the shop, police say.

"I had never found anything like this. It's the first time I found cocaine inside a soapbox," Detective Olívio Gomes Lira told the Guardian.