Police said the probe was launched after CCTV recordings were discovered on a person’s phone in a separate police probe.
The Met said all 14 “contracted workers” had been bailed until June.
A spokesperson said: “The footage shows interactions between security and members of the public on hospital grounds where force was used.”
A spokesperson for Croydon Health Services NHS Trust, which runs the hospital, said: “On 12 May, we were informed of an ongoing police investigation regarding a number of individuals employed by ICTS, who provide security services to the Trust.
“We are co-operating fully with this investigation.”
It added that it had put in place an interim security team, headed by an in-house manager, to “ensure we can continue to protect the safety of our patients and our staff.”
The Care Quality Commission, the health watchdog, said it was aware of the incident and that it would take regulatory action if “we find evidence to indicate that people are at risk”.
“CQC is aware and we are working with the trust and local police who are investigating individuals employed by a company to provide security services at Croydon University Hospital,” said a spokesperson.
“Our priority is the safety of people using services and quality of care they receive. If we find evidence to indicate that people are at risk we will always follow this up and take appropriate regulatory action if necessary to keep people safe.”
The Standard has contacted the security firm, ICTS, for comment.
The large hospital, which was previously known as Mayday Hospital, is spread over a 19-acre site, and is home to the borough’s only emergency department.