Relatives of Indian national Kirpal Singh, who was found dead under mysterious circumstances at a Lahore jail after being held for more than 20 years in Pakistan on spying charges, on Tuesday staged a protest at the Indo-Pakistan Attari border.

The relatives of Kirpal raised anti-Pakistan slogans near the Integrated Check Post at Attari border. Apart from Kirpal's sister Jagir Kaur and his other relatives, also attending the protest was Dalbir Kaur, the sister of Sarbajit Singh, the Indian death row prisoner who was killed in an attack by fellow prisoners at Kot Lakhpat jail in Lahore.

Kirpal, 50, had allegedly crossed Wagah border into Pakistan in 1992 and was arrested. He was subsequently sentenced to death in a serial bomb blast case in Pakistan's Punjab province.

He was found dead in his cell at Kot Lakhpat Jail in the wee hours of Monday.

The body has been shifted to the Jinnah Hospital in Lahore for an autopsy. Kirpal, who hailed from Gurdaspur district in Punjab, was reportedly acquitted of bomb blast charges by the Lahore high court, but his death sentence could not be commuted because of unknown reasons.