"All the accused were using their firearms illegally. Though the licence that they were carrying showed that the guns were supposed to be used for personal security, they were using it for professional purposes," a senior police official said.
Twelve rifles and dozens of cartridges were seized from them, police said.
The arrested 21 persons worked for various security agencies and all of them hail from Jammu and Kashmir, the property cell of Mumbai crime branch said.
The guards allegedly did not secure a no objection certificate from the Mumbai Police to carry weapons in Mumbai jurisdiction.
Mumbai Police sources said the drive to crack down on security guards carrying illegal fire arms was launched after Sajjad Mughal (22), a native of Jammu and Kashmir, who was employed as a security guard at 'Himalayan Heights' in suburban Wadala, allegedly killed young woman lawyer Pallavi Purkayastha on August 9 last year for resisting his advances.
According to police, Mughal used to ogle at the 25-year-old victim, the daughter of IAS officer Atanu Purkayastha, the joint secretary in the Agriculture ministry.