Six activists of former Pakistan Prime Minister Benazir Bhutto's Pakistan Peoples Party (PPP) were shot dead by unidentified gunmen late on Friday night in the town of Attock, 120 kms west of Islamabad.
"We do not know what the motive is behind the attack. We are investigating," a police spokesman Najab Khan was quoted by a foreign news agency as saying.
PPP spokesman Farhatullah Babar said the attack was aimed at scaring away party workers from preparing for the 2007 General elections, polls which Benazir is not being allowed to contest by the military-backed regime.
"We demand the arrest of the culprits. One of the men murdered in the attack was a local office-bearer (a party leader at village level)," Babar was quoted, as saying.
He said of the six dead, three were brothers.
Bhutto has lived in self-imposed exile since 1998, but has said that she will return for the election.
Pakistan President Pervez Musharraf has warned that both she and another former Prime MInister Nawaz Sharif will not be allowed to vote or to contest the elections, as both have a number of corruption-related cases lodged against them.