About 20 to 25 people, mostly party activists, were present at the office of Sardar Amjad Khosa, a member of the National Assembly, in Taunsa town in Dera Ghazi Khan district when the attack took place, killing seven people on the spot, officials said.
The attack comes two months after Punjab Home Minister Colonel (retd) Shuja Khanzada was killed in a suicide attack on his political office in Attock's Shadi Khan village.
Those killed in the attack were his close associates. People usually gather at the office to address their grievances.
"It was a suicide attack," District Police Officer Mubashir Maiken told media.
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The injured were shifted to hospital where the condition of some three of them is stated to be critical.
Tehreek-i-Taliban Pakistan splinter group - Jamaat-ul-Ahrar - claimed responsibility for the attack.
"The Jamaat-ul-Ahrar has carried out the attack on the Dera (outhouse) of the PML-N lawmaker," the militant group's spokesman Eshanullah Ehsan said in a statement to media.
Khosa condemned the "cowardly" attack and expressed the government's resolve to fight terrorism.
"Our government's resolve against terrorism will not end and the Operation Zarb-e-Azb will continue (in North Waziristan) till elimination of the last terrorist," he said.
Punjab Law Minister Rana Sanaullah said there was no police security at the outhouse as Khosa was not present there. "Security moves with the the lawmaker," he said.