Gunmen have launched simultaneous raids on two mosques of the minority Ahmadi Islamic sect in Lahore, killing more than 80 people, Pakistani police say.
The attackers fired guns and threw grenades at worshippers during Friday prayers. Three militants later blew themselves up with suicide vests.
Pakistani forces have secured both buildings, but are still searching for militants who fled the scene.
Lahore has been the scene of a string of brazen attacks.
It is unclear who carried out the attacks, but suspicion has fallen on the Pakistani Taliban, Ali Dayan Hassan of Human Rights Watch told the BBC.
Mr Hassan said the worshippers were "easy targets" for militant Sunni groups who consider the Ahmadis to be infidels.
Police said several attackers held people hostage briefly inside the mosque in the heavily built-up Garhi Shahu area.
Some took up positions on top of the minarets, and fired assault rifles at police engaged in gunfights with militants below.
Three of the attackers blew themselves up with suicide vests packed with explosives when police tried to enter the mosque, officials said.
Police were searching for at least two militants who managed to flee the scene.
Police took control of the other mosque in the nearby Model Town area after a two-hour gunfight.
Gunmen opened fire indiscriminately at the mosque, before security forces managed to kill one militant and capture two others, eyewitnesses told the BBC.
They were said to be armed with AK-47 rifles, shotguns and grenades.
Sectarian attacks have been carried out by various militant groups in Punjab province, and across Pakistan in the past.
While the Ahmadis consider themselves Muslim and follow all Islamic rituals, they were declared non-Muslim in Pakistan in 1973, and in 1984 they were legally barred from proselytising or identifying themselves as Muslims.
Members of the community have often been mobbed, or gunned down in targeted attacks, says the BBC's M Ilyas Khan in Islamabad.
But this is the first time their places of worship have suffered daring and well co-ordinated attacks that bear the mark of Taliban militants, our correspondent adds.
The London-based Ahmadi association said the attacks were the culmination of years of "unpoliced persecution" against the Ahmadis.
"Today's attack is the most cruel and barbaric," the Ahmadiyya Muslim Community UK said in a statement.
The Chief Minister of Pakistan's Punjab province, Shahbaz Sharif, expressed "heartfelt sorrow" over the killings.
"No condemnation, however strong, will be enough for these incidents," he said.
US state department spokesman Philip Crowley said Washington also condemned the "brutal violence against innocent people".