Indian national Sarabjit Singh, currently on death row in a jail here in Pakistan, today suffered head injury and was admitted to a hospital after he was attacked by a fellow prisoner.
The attack reportedly occurred while Singh was being moved from one cell to another.
He sustained a head injury and was initially taken to the hospital within the prison.
After his condition worsened, Sarabjit was taken in an ambulance to Jinnah Hospital. There was no official word on the incident.
Officials of the Indian High Commission told PTI that they had not been informed by Pakistani authorities about the incident.
The Kot Lakhpat jail currently has some 17,000 prisoners though its official capacity is only 4,000.
There have been instances in the past of prisoners being attacked and even killed within the prison.
Authorities had tightened Sarabjit's security after the recent execution in India of Afzal Guru, who was convicted of involvement in the 2001 terror attack on the Indian parliament.
Sarabjit was convicted for alleged involvement in a string of bomb attacks in Punjab province that killed 14 people in 1990.
His mercy petitions were rejected by the courts and former President Pervez Musharraf.
The outgoing Pakistan Peoples Party-led government put off Sarabjit's execution for an indefinite period in 2008.
Sarabjit's family says he is the victim of mistaken identity and had inadvertently strayed across the border in an inebriated state.