The shooting was the latest in a string of violent attacks against Christians and Westerners, who have been increasingly targeted since Pakistani President Gen. Pervez Musharraf's decision to crack down on Islamic extremist groups and join the U.S. war against the Taliban and al-Qaida in neighboring Afghanistan.
The killings occurred at the third-floor offices of the Institute for Peace and Justice, or Idara-e-Amn-o-Insaf, a Pakistani Christian charity that does work in the city. Victims were tied up in chairs with their hands behind their backs and their mouths taped before being shot point-blank in the head, according to Karachi Police Chief Kamal Shah.
All seven of the dead were Pakistani Christians, contradicting earlier police reports that three of the victims were Muslim. One worker who survived the attack later died in a hospital, police said. It was not clear who was behind the attack.
Shah said police found eight empty shell casings, one for each of those shot. He said five of the dead were found seated in a main room at the office, and the sixth was tied to a chair in the bathroom. He said police are questioning an office assistant who was tied up and beaten by the attackers, but not shot.
Police want to know how the gunmen got into the office, which had an electronic door that could only be opened from the inside, he said. The office assistant has told police there were two gunmen involved in the shooting, Shah said.
By late morning, hundreds of police had cordoned off the 13-story building in a central business district of Karachi.
A female relative of one of the victims arrived at the office, sobbing and beating herself in anguish before being led away by police.
The Christian group has been in operation for 30 years, working with poor municipal and textile workers to press for basic worker rights, and organizing programs with local human rights groups.
Pakistani Information Minister Nisar Memon condemned the attack, saying those who carried it out were ``enemies of Pakistan.''
``We are particularly sad about the killings in Karachi because the terrorist have targeted unarmed Christian civilians,'' Memon told The Associated Press. He added that the ``cowardly terrorist attacks'' would not deter Pakistan's resolve.
``Pakistan's cooperation with the world community in the war against terrorism will continue,'' he said.
The violence shattered a growing sense of confidence among Pakistani leaders that a wide-sweeping crackdown had broken the back of extremist groups that have targeted Christians and Westerners.
This month, police in Karachi have arrested 23 members of Harakat ul-Mujahedeen Al-Almi, which is believed behind a June bombing outside the U.S. Consulate, a suicide car bomb in May that killed 11 French engineers, and aborted plots to attack U.S.-based restaurants McDonald's and KFC in the city.
Musharraf touted police efforts against the group, saying the success was the reason there had not been any serious attacks in Pakistan in recent weeks.
``All the first-string operators are behind bars except a very, very few,'' he said in an interview earlier this month with The Associated Press.
A string of violent assaults on Christian organizations have killed at least 36 people and injured about 100 since Musharraf's decision to join the U.S. war in neighboring Afghanistan.
On Aug. 9, attackers hurled grenades at worshippers as they were leaving a church on the grounds of a Presbyterian hospital in Taxila, about 25 miles west of the capital, Islamabad. Four nurses were killed and 25 other people wounded. Two men alleged to have supplied guns and grenades to the attackers were arrested in recent days, police said Tuesday.
Four days before the Taxila attack, assailants raided a Christian school 40 miles east of Islamabad, killing six Pakistanis including guards and non-teaching staff.
And on March 17, a grenade attack on a Protestant church in Islamabad's heavily guarded diplomatic quarter killed five people, including an American woman, her 17-year-old daughter and the lone assailant.
During the arrest of two of the Al-Almi militants, police found maps of two churches and a Christian school in Karachi, along with weapons and explosives, Interior Ministry officials have said.
That discovery prompted authorities nationwide to remove signs from around some churches set up in private homes and to fortify other Christian sites with sandbag bunkers.