(Reuters) - Chicago Police arrested 50 people for using "secret groups" on Facebook to sell guns and drugs, the police said on Thursday, and they criticized the world's biggest social media network as being unhelpful during the 10-month investigation.

Police did not detail the charges filed against the 50 men and women arrested through Thursday, but said there were "dozens and dozens" of the secret groups. Arrest warrants for 18 more have been signed and most of those affected have criminal histories, police said.

Chicago Police Superintendent Eddie Johnson, at a news conference to announce the arrests, criticized Facebook for not helping police.

"Facebook has a responsibility to the people that they serve to ensure that these types of things don't go on," Johnson said. "And, quite frankly, they haven't been very friendly to law enforcement to prevent these things."

    Johnson expressed hope that public attention will pressure Facebook to work to quell criminal activity by some of its 2 billion users worldwide.

"Certainly they can spend a little money to ensure that these type of things don't happen," Johnson said. 

In an emailed statement, Facebook Inc said, "We are investigating this report now as it is the first time we have been alerted to it.

"We do not allow the sale of guns or drugs on our platform. We routinely work with law enforcement and outline how officials may submit a request on our site," the company added.

Anthony Riccio, chief of the Chicago police organised crime unit, declined to say whether investigators have been in contact with the company, saying that was "ongoing."

Among those arrested was an elementary school teacher taken into custody at his Chicago school in possession of scales often used for weighing drugs, Riccio said.

Since a confidential informant alerted investigators about the alleged criminal trade on Facebook in February, police arranged the purchase of 17 different types of drugs and 18 different illegal firearms, Riccio said.

Chicago has been singled out by President Donald Trump as one of the most violent U.S. cities. In 2016, murders there exceeded 760.