Jagtar Singh Johal was arrested by plain-clothed police officers in the Punjab region in early November, just weeks after he travelled to India to get married.
Authorities in the country said the 31-year-old from Dumbarton was being investigated under several counts of aiding and abetting of murder, allegations both he and his loved ones have denied.
Despite the fact Mr Johal remains in custody, no charges have been brought against him, prompting renewed criticism from human rights organisations.
Concerns have been raised about reports of him being subject to abuse, mistreatment and physical torture during his time in custody.
His brother, Gurpreet Singh Johal, described Mr Johal - known to his friends and loved ones as Jaggi - as a peaceful activist who had contributed to a website remembering the 1984 massacre at the Golden Temple at Amritsar, but said he was not a militant.
Gurpreet, a solicitor, said: “The past few months have been incredibly difficult. The longer Jagtar spends in custody without charge, the more concerned we are about its impact on his physical and mental wellbeing.
“It was Jagtar’s birthday on Friday and we are heartbroken that he was forced to spend it imprisoned and away from his loved ones. We are determined to bring Jaggi home so his next birthday is enjoyed here alongside his friends and family.”
Mr Johal’s wedding took place on 18 October and he had been due to travel back to Scotland on 1 November with the rest of his family. However, he decided to stay on in India with his new wife, who needed to apply for a visa to enter the UK. He was later arrested on 4 November while the couple were out shopping.
Martin Docherty-Hughes, the SNP MP for West Dunbartonshire, has been leading cross-party efforts to ensure Mr Johal’s rights under international law are protected.
He said: “It’s now 100 days that my constituent has been held by the Indian authorities and he has yet to be charged with any crime.
“Jagtar’s family in Dumbarton are deeply concerned for his welfare. He is their son, their brother and their friend – and they remain determined to secure justice for Jagtar and bring him home to Scotland.
“It’s critical that the family receives the full backing of the UK government. I will continue doing all I can to support the family and ensure Jagtar is treated fairly and afforded an open and transparent judicial process.”
Rupert Skilbeck, director of the human rights group, Redress, said: “We remain seriously concerned about Jagtar’s treatment in detention, and it is crucial that the UK does everything in its power to ensure that his rights are protected and any torture is immediately stopped, that he is provided with any medical care needed, and that he receives proper due process.”