An American woman and her daughter were among five people killed when an attacker tossed grenades inside a Protestant church during a Sunday service in the diplomatic quarter of the Pakistani capital.
There was no claim of responsibility but suspicion fell on hardline Islamic groups opposed to President Pervez Musharraf's support for the US-led "war on terror" after the Sept. 11 attacks on the US.
The 60 to 70 people at the Protestant International Church, a popular place of worship for foreigners in Islamabad, had sung some hymns and were listening to the sermon.
The spiritual calm was shattered by a blast at the back of the hall and one man rushed up the aisle brandishing grenades and shouting, witnesses said.
Worshippers dived for cover as five or six explosions ripped through the church, filling it with smoke and splattering the walls and ceiling with blood.
A government statement said a lone attacker was responsible for killing the five churchgoers -- two Americans, one Pakistani, one Afghan and an unidentified person -- and wounding 42.
The death toll could rise as the government said six or seven people were gravely wounded.
Musharraf -- who has banned seven militant groups and ordered the detention of hundreds of activists since Sept. 11 -- called the attack a "ghastly act of terrorism," according to the state news agency.
Nick Parham, a Briton who works for the Tearfund aid agency, said he saw the attacker at close range shortly after the first blast.
"One chap came down the aisle a couple of feet away from me. He had a belt on with a whole load of what looked like British army smoke grenades or homemade grenades," Parham said.
"He had one in his hand. At that point I hit the deck. There were five or six more explosions."