A South African prison run by the British security company G4S is under investigation for allegedly using forced injections and electric shock treatment to subdue inmates.

Prisoners, warders and health care workers said that involuntary medication was regularly practised at the Mangaung Correctional Centre near Bloemfontein. G4S denies any acts of assault or torture.

The revelations come just weeks after the South African government took over operations from G4S after finding it had "lost effective control over the prison" in the wake of a series of stabbings, riots, strikes and a hostage taking.

The latest allegations follow a year-long investigation by the Wits Justice Project (WJP) – part of the journalism department of the University of Witwatersrand – which uncovered damning video evidence apparently showing forced medication. A staff member at the prison hospital, who did not wish to be named, alleged that inmates were injected with Clopixol Depot, Risperdal, Etomine and Modecate. These anti-psychotic drugs can cause memory loss, muscle rigidity, strokes and other serious, potentially life-threatening side effects.

A video shot on 24 May by the prison's emergency security team (EST), which is legally required to film all its actions, shows inmate Bheki Dlamini being injected involuntarily. "I am not a donkey," Dlamini protests loudly, yelling: "No, no, no," while five men grab him, twist his arms behind his back and drag him to a room where they wrestle him on a bed and a nurse is called.

The staff member said Dlamini's medical files do not indicate he is psychotic or schizophrenic. Egon Oswald, a human rights lawyer representing Dlamini, said: "He told me that he got into an argument with a warder about the prison food and the EST was called."

Fourteen recently dismissed EST members, who spoke to the WJP on condition of anonymity, said they would restrain inmates so they could be forcibly medicated up to five times a week.

The EST members said they had no idea what the inmates were injected with. They claimed that inmates with psychiatric problems or who were being difficult or aggressive received the involuntary treatment.

Inmates also alleged that they were subjected to electric shocks by prison officials. Former inmate Thabo Godfrey Botsane was held in a single cell for four months in 2009. He claims the security team visited him one day because a cell in his unit had been set alight. "They stripped me naked, poured water over me, electroshocked and kicked me. They left me naked and bleeding on the floor. A guy from the prison intelligence unit – not a nurse – came back and he injected me in my buttocks."

Former warders Pule Moholo, Dehlazwa Mdi and Themba Tom worked in a block at the prison with single cells, known as Broadway. All three say they remember the sound of inmates screaming. Tom said: "There was a sound-proof room called the 'dark room'. EST members would bring inmates there, strip them naked, pour water over them and electroshock them. We would try not to hear the crying and screaming. It was awful." G4S denies the room's existence.

A former G4S employee, who did not wish to be named, said electric shields were necessary because he and his colleagues were hopelessly outnumbered by dangerous prisoners. "We use them sometimes because we are understaffed and we are expected to bring out the results and also to install fear on the inmates," he told the BBC. "We went overboard, so to say: sometimes you go and shock them individually in a segregation unit just to make sure they could be afraid of us."

"The management was very happy with the results and with some of the incidents if it was during the week then the official was there at the centre and they would respond with us and we do these things with them, in their presence," he said.

He admitted using an electric shield on inmates to make them talk. "Yeah, we stripped them naked and we throw with water so the electricity can work nicely … Again and again. Up until he tell you what you want to hear, even if he will lie, but if he can tells you what I want to hear. He can tell the truth but if that's not the truth that I want, I will shock him until he tells the truth that I want even if it's a lie."

Asked by the BBC interviewer if using electroshocking to get answers from prisoners constitutes torture, the EST member replied: "Yes in a simple way … Yes it was common practice."

British law firm Leigh Day – which recently secured compensation for Kenyan victims tortured by British colonial forces – has been instructed to investigate the claims against G4S in the UK.

Mangaung Correctional Centre is the second largest private prison in the world, and 81% of shares are owned by G4S Care and Justice, one of G4S's three operating companies.

Sapna Malik, partner at Leigh Day, said: "The allegations raised are shocking in the extreme and require urgent and thorough investigation. If proved to be true, prompt restitution, accountability and lessons learned must follow."

Egon Oswald added: "We have signed affidavits of five inmates who allege that they have been injected and we think more will come forward. My firm is collaborating with Leigh Day to litigate their claims."

Forced medication is subject to stringent rules in South Africa. The head of a health institution can decide to treat a patient involuntarily if two clinicians have assessed the patient and if a family member, guardian – or if they are unavailable, a health worker – has approved. Involuntary medication is then only permissible if the patient is a danger to himself or others and if he is incapable of making an informed decision.

South Africa's correction services minister, Sibusiso Ndebele, said an investigation would be launched into the allegations, saying he viewed them "in a very serious light".

"We will leave no stone unturned in this investigation, in order to ensure that those implicated in such inhumane acts face the consequences of their actions," he said on Friday.

G4S denied any acts of assault or torture, either by means of electroshocking or medical substances, against inmates. "G4S has a zero tolerance policy against the use of undue or excessive force," it said. "Inmates have unrestricted and confidential access to the DCS controller, employees from the office of the inspecting judge, the director, healthcare personnel and psychologists, with whom they can log complaints and raise concerns. Should any laws have been broken, DCS would have strongly acted against G4S."

Andy Baker, the president of G4S Africa, denied ny abuse had taken place and said inmates were given injections if they required medication "for their own good".

"It is important to note that the G4S people do not make the decision to medicate, the medical staff do not work for G4S, they are a completely independent entity," he told the BBC.

G4S was awarded a 25-year contract in 2000 for the construction, maintenance and running of the jail. The DCS will hand back the prison if and when G4S can prove its ability to run it again.

• This article was amended on 28 October 2013. Andy Baker's denial initially read, "It is important to note that the G4S people do make the decision to medicate …". It has been corrected to, "It is important to note that the G4S people do not make the decision to medicate …".