The male friend of 23-year-old Indian student who died after being gang-raped on a bus in Delhi speaks for first time about the attack. ITN

The men accused of raping and murdering a 23-year-old physiotherapy student in Delhi last month were tortured by police after their arrests, their lawyer claims.

Manohar Lal Sharma, who represents three of the six men arrested two days after the incident, told the Guardian that all those on trial for the attack were "brutally assaulted" by police during interrogations.

Five men aged between 19 and 35 face a potential death sentence for their alleged roles in the attack on 16 December, which led to widespread outrage and provoked massive protests across India. "They were beaten most brutally. They were forced to drink urine and from the toilet. They were sexually assaulted with sticks in their backside. Whatever statements they made were made under duress and are worthless," Sharma said.

A Delhi police spokesperson said he had no comment on the allegations.

Indian police face frequent accusations of abusing detainees and have been repeatedly criticised by campaign groups for systematically resorting to violence during interrogations. "Torture in custody is so widespread that this is what defence lawyers always say. Their claims are taken seriously because it is almost routine that detainees are subject to beating or worse," said Meenakshi Ganguly, South Asia director of Human Rights Watch.

Though there will be little sympathy for the claims of brutality in India, the accusation is likely to further raise concerns abroad over the conduct of their trial.

Local legal experts have already pointed out that the accused did not have legal representation during their police interrogations. Under Indian law only statements made before a magistrate can be admitted as evidence in a trial.

There are fears that the massive public pressure to secure rapid convictions and hangings could lead to procedural short-cuts.

"The worry is this becomes a search for the quickest way to hang these guys. A trial is a performance for the public in which the state shows that it can protect its citizens. But, especially in a case so egregious and so brutal it is very important to have a fair trial," said Maneka Guruswamy, a supreme court lawyer in Delhi.

However, Guruswamy said that with the victim's friend who was with her throughout the attack available as a witness, as well as DNA evidence linking the accused to the crime and a statement taken from the victim before she died, there was less incentive for police in this case to use violence to extract confessions.

The trial, held in a new fast-track court set up to deal with rape and related offences, is being held behind closed doors after chaotic scenes in an earlier hearing. A 17-year-old boy will be tried separately in a juvenile court.

The accused, who include a bus driver, a cleaner and a part-time gym instructor, are accused of luring the woman and her male friend on to a bus at 9pm. The pair were returning from watching a film. After being repeatedly assaulted, they were dumped on a roadside. The woman, who has not been named in local media, died two weeks later in a Singapore hospital from internal injuries. She had been repeatedly assaulted with an iron bar.

Sharma, the lawyer, said he had met his three clients in Delhi's Tihar high-security jail on Wednesday. "They will plead not guilty. They are innocent. They are mourning," he said.

Two of the defendants are reported to have said they want to be state witnesses. Police said they would not accept the request, according to local media.

New details have also emerged about the 17-year-old defendant. Official documents describe an illiterate teenager who dropped out of school aged 12, ran away from home and then spent five years shifting between menial temporary jobs in Delhi, in hotels and a dairy, before being befriended by Ram Singh, the alleged ringleader of the accused, while working at a roadside restaurant.

The teenager, the oldest of five children, has said he had travelled to the slum where Singh lived with his brother, also among the accused, on the day before the attack to give him some money, the Guardian has learned. He stayed the night and cooked chicken for the Singh brothers during a drinking session the next afternoon. According to official documents he told a magistrate who took his statement after his arrest that he had "gone mad for a moment" and was "very sorry".

Amid an unprecedented debate over cultural attitudes to women, Indian authorities have responded to criticism by proposing a range of measures to improve safety, including more CCTV cameras in city centres and "gender sensitisation" lessons for schoolchildren.

Continuing reports of other attacks on women around the country — few of which would have received much attention a month ago — underline the scale of the problem. Official data shows one rape is reported on average every 20 minutes in India.