Jagtar Singh Johal, 31, travelled to India in October last year to marry his fiancée, but a fortnight later was detained in Punjab by plain clothes officers while he was out shopping.
His lawyers say he was placed under a hood, taken to an undisclosed location and tortured for several days, before being forced to sign blank sheets of paper.
Mr Johal told them he had electric shocks applied to his ears, nipples and genitals, had his limbs forced in opposite directions and was prevented from sleeping.
Six months after the alleged torture, he has still not been granted an independent medical examination despite repeated requests by his legal team.
Mr Johal, from Dumbarton, is one of ten men accused of being involved in the murder of Hindu nationalist leader Ravinder Gosain, who was shot dead in October.
At a court hearing in the Punjabi city of Mohali yesterday, prosecutors said the charge sheet against the men ran to nearly 1,500 pages and involved 172 witnesses.
However, it is unclear how much of the evidence relates to Mr Johal, with human rights groups stressing that any material obtained through torture must be inadmissible.
“There are extremely serious allegations that Jagtar has been tortured,” said Rupert Skilbeck, director of Redress, which has been providing Mr Johal’s family with legal assistance.
“The government of India needs to investigate these allegations immediately. Any evidence obtained through torture cannot be used by the court.
“If Jagtar has indeed been tortured, the credibility of the entire legal process against him is undermined.
“This is why it is crucial that he is given an independent medical examination and that an immediate investigation into the allegations of torture is opened.”
Supporters of Mr Johal, including members of the Sikh Federation, took part in a demonstration outside the UN building in Geneva yesterday.
Mr Johal’s brother Gurpreet Singh, who attended the protest, said the family were still unclear about the exact nature of the charges.
“My understanding of it is that they had all this ‘proof’ on 7 November – so why has it taken them 180 days to provide this charge sheet?” he said.
“Jagtar’s torture claims have still not been investigated, and any of the evidence obtained during the torture period would be invalid anyway, including any blank documents that Jagtar has told us he has signed.”
He added that he had last spoken to his brother in February after the British High Commission facilitated a short phone call lasting only a few minutes.
The Indians authorities have maintained that due process is being followed and the Punjabi police have denied allegations of torture.