Indian death row prisoner Sarabjit Singh died of cardiac arrest in a Lahore hospital in the wee hours on Thursday after being comatose for nearly a week following a brutal assault by fellow inmates in a high-security Pakistani jail, officials said.

'I received a call from the doctor on duty (at Jinnah Hospital) at 1am (1:30 IST) informing me that Sarabjit is no more,' Mahmood Shaukat, the head of a medical board that was supervising 49-year-old Sarabjit's treatment, told.

Another doctor, who was part of the team treating Sarabjit, said he died of cardiac arrest, adding that doctors made several unsuccessful attempts to resuscitate him.

Officials of the Indian high commission in Islamabad said they had been informed by officials of Jinnah Hospital about Sarabjit's death.

Sarabjit sustained severe injuries when at least six prisoners attacked him in a barrack at Kot Lakhpat Jail on Friday, hitting him on the head with bricks.

In New Delhi, Prime Minister Manmohan Singh expressed sadness over Sarabjit's death, saying criminals responsible for the barbaric and murderous attack on the Indian national must be brought to justice.

Sarabjit was convicted of alleged involvement in a string of bomb attacks in Punjab province that killed 14 people in 1990 and spent about 22 years in Pakistani prisons.

His family, who had just returned to India after vis.