Christians in this overwhelmingly Muslim country went to worship services Sunday in what has become an act of bravery and, for some, faithful defiance in the face of deadly attacks by Islamic extremists bent on revenge for the U.S.-led war on terrorism.

In the capital, Islamabad, Protestant worshippers filed into St. Thomas Church past patrols of motorcycle police, security guards with metal detectors and female security agents checking women's bags.

In his sermon, Rev. Irshad John told the congregation they should not fear death, but later asked them not to gather in groups outside the church gates once the service was over. That could make them a target, he said, and "that could be a problem for all of us."

Funmi Opatola, 49, is the wife of Nigeria's deputy ambassador to Pakistan and a lector at the St. Thomas parish.

"People feel threatened. You don't know what's going to happen," she said.

Her voice booming from the lectern, Opatola called for worshippers to "pray for those who are too scared to come out of their homes. Give them the strength and the courage to come out."

A random sampling of churches in Islamabad on Sunday suggested that they were filling up with people.

The pastor of a Pentecostal church politely asked an American reporter not to show up at Sunday services, saying the presence of a Westerner could put the whole congregation at even greater risk.

At another Protestant church, Rev. Khalid Pervaiz said Pakistani Christians were surprised they had become a target.

"People are feeling sad and sorry and they are feeling insecure. ... We were thinking that revenge might come after the war started in Afghanistan but we were not expecting to take it out on the Christians," he said.

But the militants have found Christians and their institutions inviting targets in their anger over President Gen. Pervez Musharraf's having sided with the United States against the Afghan Taliban and Al Qaeda terrorists after the Sept. 11 attacks on New York and the Pentagon.

Last Monday, armed assailants burst onto the grounds of the Murree Christian School, killing six Pakistanis before escaping. The school is for the children of Christian missionaries working in Pakistan. None of the children was harmed and all those killed were local employees of the school.

On Friday, three men threw grenades at nurses leaving a church service on the grounds of a Presbyterian hospital in Taxila, killing three nurses. A fourth nurse died early Sunday of her injuries.

Last week's attacks were only the latest in a string of violent assaults on Christian organizations that have killed 30 people and injured about 100 since Musharraf gave his support to the U.S.-led anti-terror campaign.

Christian leaders say Islamic extremists are targeting them because they associate Christianity with the West, especially America and Britain, the main protagonists in the war against the Taliban and Al Qaeda. Christians make up about 1.5 percent of Pakistan's 145 million people.

Representatives of all of Pakistan's minority religions, meanwhile, have demanded government action to stop the violence. And while the government has condemned all the attacks, Friday's violence against the hospital seemed to hit a particularly raw nerve.

The hospital is supported by the U.S. Presbyterian Church and the Presbyterian Church of Pakistan, but mainly has treated poor Muslim patients since 1922.

In a statement Sunday, an organization of mainstream Muslim clerics blamed the violence on "anti-Islam and anti-Pakistan elements" and said it "shows that there is a criminal conspiracy against Pakistan."

Senior Interior Ministry officials believe the most recent attacks were carried out by members of Jaish-e-Mohammed, one of several organizations banned by Musharraf and believed to have links to Al Qaeda.

Jaish-e-Mohammed has traditionally been involved in the guerrilla war against the Indian army in Indian-controlled Kashmir.

The All Pakistan Minorities Alliance, which represents all religious minorities including Christians, Sikhs and Hindus, called on Sunday for Interior Minister Moinuddin Haider to resign for failing to curb the violence or protect Pakistani Christians.

"The government was receiving reports that something could happen, but it did not provide any security," said Alliance chairman Shahbaz Bhatti. "Why did they not take action?"

Alliance leaders said they would stage strikes and other peaceful protests Thursday to press their demands.

The leaders demanded that the government provide tight security for Christian organizations and all minorities, including Christian lawyers who receive threats for defending their clients against the country's Islamic-oriented blasphemy laws.

They also called on the world's major powers, especially the United States, to do what could be done to protect Christian minorities in Pakistan.

"We are suffering because of the actions the U.S. is taking" in the war against terrorism, said Pritam C. Patrick, a staffer at the Presbyterian hospital whose wife was injured in Friday's attack. "The U.S. must do something to protect us."