Among the dead are three policemen, a woman and a private security guard, Sajjad Bhutta, a government official, told Reuters.
The attackers fled after the attack, officials said.
Sohail Sukhera, a police official, said: "We are tracing the attackers. We will not spare them."
At least 30 injured worshippers and one of the alleged perpetrators of an attack on two mosques of a minority religious community in Lahore were being treated in Jinnah hospital when it was attacked.
Eighty-two people were killed in the attacks on Friday, which targeted Ahmadiyya mosques
Imran Khan, Al Jazeera's correspondent in Islamabad, the Pakistani capital, said, quoting the local media, that the assailants were trying to rescue the hospitalised attacker when Monday's fighting erupted.
The reports also said the attackers were dressed as police, wearing uniforms, he said.