They entered the third-floor offices of the Institute for Peace and Justice (IPJ) and shot their victims in the head.

One other person is in a critical condition.

All those killed were Pakistani Christians.

Both gunmen escaped after the attack in the southern port city which has seen mounting attacks this year on Western and Christian targets.

Police have cordoned off the 12-storey building housing the offices.

Karachi police chief Tariq Jamil said the victims had their hands tied and their mouths had been covered with tape.
The All Pakistan Minority Alliance - a grouping of minority political parties - says it is planning three days of mourning in protest against the latest attack against the Christian community.

"Christians will march on the United Nations in Islamabad to appeal to the international community for help," the head of the alliance, Shahbaz Bhatti told BBC News Online.
"We have become increasingly victimised since the launch of the US-led international war on terror. It is, therefore, the responsibility of the international community to ensure that the government protects us."

Work among the poor

The BBC's Zaffar Abbas reports that the IPJ has never taken part in religious politics in Pakistan.

Run by left-wing Christians, it has focused on helping the poorest sections of society.

It has been operating for the past 30 years, working to obtain basic employment rights for municipal and textile workers and running programmes with local human rights groups.

Police have not said who they suspect is behind the latest killings, but the authorities believe a string of attacks on Christian and Western targets have been carried out by militants opposed to the US presence in neighbouring Afghanistan.

Murder

The Archbishop of Karachi, Simeon Pereira, told BBC News Online that it was too early to tell who carried out the killings but said they could be linked to a "grudge".
He said police were exploring all avenues, including a possible link with the murder of the charity's former chairperson, Ivan Moon.

Mr Moon died four months ago in mysterious circumstances. He was found dead in his office after being injected with poison.

"I don't believe this was a terrorist attack," the archbishop said. "There has been trouble at the institution before. I think the killings are linked to the murder of Mr Moon."

But he said the attack would undoubtedly raise fears within the Christian community of further violence.

"We have police posted outside our churches day and night. The Christian community is not panicking at the moment, but I don't know what will happen as a result of the latest killings - people will suspect it was a terrorist attack."
A witness quoted by Reuters news agency said the gunmen were clean-shaven and wearing shirts and trousers.

Rising violence

Wednesday's shooting appears to be the first attack on Christians in Karachi, our correspondent notes.
Karachi witnessed an attack on the US consulate in June and a suicide bombing against French naval engineers in May.
The authorities also say militants were plotting to assassinate President Musharraf when he visited the city, and had plans to target US-based fast-food restaurants.
Attacks in the past year on Christians elsewhere in Pakistan have left at least 30 people dead. They include:

· An attack on a missionary school in Murree which left six dead
· An attack on a hospital in Taxila which killed three people
· A grenade attack on a church in Islamabad in which five people died
· A gun attack on a church in Punjab province in which 18 worshippers died

When police arrested 21 members of the Islamic militant group, Harakat ul-Mujahideen Al-Almi, this month in connection with the consulate attack, they found plans of Christian facilities in Karachi.

As a result, the authorities fortified some of the sites with sandbags and removed signs from others.