The attack hit the city of Peshawar, which is on the outskirts of Pakistan's tribal area, the main sanctuary for Islamic militants. They have targeted the city with scores of bombings in recent years.
Three militants initiated the attack on the mosque, located inside a Shiite religious school, by firing on a policeman who was standing guard outside, said senior police official Shafiullah Khan. The policeman was critically wounded, Khan said.
The militants then entered the mosque, where one of them detonated his suicide vest. The other two militants escaped, and police have launched a search operation to find them, Khan said. Fifteen people were killed and scores more wounded, he said.
Zawar Hussain, who was inside the mosque when the attackers struck, said their firing set off panic among the roughly 300 worshippers inside. Then came the explosion.
"After the blast, I fell down. People were crying for help," said Hussain. "I saw bodies and badly injured worshippers everywhere."
Local TV video showed blood splattered on the floor and walls of the mosque. Broken glass littered the floor, and there were holes in the walls and ceiling caused by ball bearings packed in with the bomber's explosives to cause maximum damage and casualties. Relatives at a local hospital wailed in grief as rescue workers wheeled in wounded victims, their clothes soaked in blood.
No one has claimed responsibility for the bombing.
Radical Sunni Muslims who consider Shiites to be heretics have stepped up attacks against the minority sect in Pakistan over the last several years.
On Saturday, a bomb that appeared to be targeting Shiites ripped through a bus carrying female university students in the southwest city of Quetta, killing 14 people. Militants then attacked a hospital where wounded victims were taken, killing more people.
The militant group Lashkar-e-Jhangvi claimed responsibility for the attack in Quetta and could be suspected in Friday's Peshawar attack as well. The group has carried out many of the attacks against Shiites in Pakistan in recent years, especially in Baluchistan province, where Quetta is the capital.
Although most Sunnis and Shiites live peacefully together in Pakistan, the country has a long history of sectarian attacks by radicals on both sides.
In the 1980s and 1990s, Pakistan became the scene of a proxy war between mostly Shiite Iran and Sunni Saudi Arabia, with both sides funneling money to sectarian groups that regularly targeted each other.
Most of the attacks in recent years have been by radical Sunnis against Shiites. Last year was one of the most deadly for Shiites in Pakistan's history, according to Human Rights Watch, which said more than 400 Shiites were killed.
This year is shaping up to be even deadlier. Two attacks carried out by Lashkar-e-Jhangvi in Quetta at the beginning of the year killed nearly 200 people.
The sectarian violence presents a significant challenge to Pakistan's new government, which took power earlier this month under the leadership of Prime Minister Nawaz Sharif.
sHuman rights activists and members of the Shiite community criticized the last government for failing to do enough to stop the attacks. The new government has promised to do more, but some critics have questioned whether Sharif will follow through. His party has done little to crack down on Lashkar-e-Jhangvi and other militant groups in its home province of Punjab in central Pakistan, even though the party controlled the provincial government for the last five years.
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