Manish Tewari, India's information and broadcasting minister, said his government had been pressing Pakistan to release Singh since 2005.
External Affairs Ministry spokesman Syed Akbaruddin said India demands a thorough investigation to identify who was responsible for the attack.
"This was simply the killing of an Indian citizen while in the custody of Pakistani authorities," the statement said. The attack "highlights the need for a concerted action by Pakistan to safeguard Indian prisoners in Pakistan."
AP Writers Sebastian Abbot and Zarar Khan in Islamabad, Pakistan, contributed to this report.