An Indian academic detained for almost six years without trial has been granted bail by the Supreme Court, highlighting the country's use of harsh anti-terror laws decried by rights activists.

Shoma Sen, 66, a former professor of English at Nagpur University, was arrested in 2018.

She was one of 16 activists and academics held for allegedly inciting violence between different Indian caste groups, among them a Jesuit priest, Stan Swamy, who died in pre-trial detention three years later at the age of 84.

The National Investigation Agency (NIA), the country's top anti-terror agency, also claimed that Sen and some of the other activists had links with far-left Maoist insurgents.

New Delhi has been battling armed Maoist rebels, known locally as Naxals, for decades in dense, tribal-dominated forests of central and eastern India.

"At present, the appellant has been in detention for almost six years, her age is over 66 years and charges have not yet been framed," the two-judge bench said on Friday.

"If we examine the acts attributed to the appellant by the various witnesses or as inferred from the evidence... we do not find prima facie commission or attempt to commit any terrorist act" by Sen, the court added.

Another sexagenarian activist detained in the same case, Sudha Bharadwaj, was released by the Mumbai High Court in 2021.

Sen, like Bharadwaj and Swamy, was held under the Unlawful Activities Prevention Act (UAPA), which allows indefinite detention without trial.

Critics say the law -- which makes it difficult for accused people to receive bail -- has been used by Prime Minister Narendra Modi's government to silence dissent.

According to Indian government statistics, more than 1,000 people are detained under the UAPA each year, while fewer than 100 are convicted.