Pakistani cabinet ministers on Tuesday opened negotiations with Shiite Muslim protesters who are demanding army protection and refusing to bury victims of a bomb attack that killed 89 people in Quetta.
Saturday's attack, the second bomb targeting the Shiite Hazara minority in five weeks in the southwestern city, has brought hundreds of protesters onto the streets across the country and shut down parts of Karachi, Lahore and Islamabad.
Shiites, who make up around 20 percent of the mostly Sunni Muslim population of 180 million, are facing record numbers of attacks, raising serious questions about security as nuclear-armed Pakistan prepares to hold elections by mid-May.
Lashkar-e-Jhangvi (LJ) has claimed responsibility for the attacks and Shiites are furious that authorities have done nothing to prosecute those responsible.
Prime Minister Raja Pervez Ashraf sent ministers to Quetta to negotiate with the protesters and ordered an "immediate launch of a targeted operation" against those responsible, his office said without elaborating.
Local officials in Quetta announced earlier Tuesday that security forces had killed four men and arrested seven including an alleged mastermind of Saturday's bombing in an "ongoing" raid on the edge of the city.
Police said another 172 people had been rounded up in the surrounding province of Baluchistan in the past two days, including the provincial chief of extremist Sunni outfit Ahle Sunnat Wal Jamaat, and they were now being questioned.
Pakistani security forces frequently detain people en masse after major bombings but few if any are ever charged. In 2011 a court released on bail the head of LJ, Malik Ishaq, even though he has been implicated in dozens of murders.
The president of the Shia Ulema Council, Sajid Naqvi, warned the protests could intensify if they were not happy with Tuesday's meeting with ministers.
"The government must know that if the talks fail the situation in the country will further deteriorate," he told AFP.
But Daud Agha, chairman of the Baluchistan Shia Conference which is one of the groups taking part in the protest, expressed satisfaction at the raid and said a time would be announced for the burials to begin.
Another senior Shiite leader, Sikandar Gilani, said the matter could be resolved if the army gave a guarantee they would launch an operation against the extremists.
Thousands of Shiites maintained their protest vigil in Quetta for a third day on Tuesday, refusing to bury the dead until the authorities take action.
In the courtyard of a Shiite mosque near the protest site, scores of coffins lay in neat rows, most decorated with pictures of the victims from Saturday.
Relatives sat with the coffins reciting verses for the dead from the Koran. The youngest victim was a five-year-old boy who died along with his mother when the bomb, containing nearly a tonne of explosives, demolished a shopping centre.
A similar protest, after suicide bombers killed 92 people at a Hazara snooker hall in Quetta on January 10, lasted for four days until Islamabad sacked the provincial government and imposed governor's rule.
Quetta is a small town where the military and intelligence agencies have a heavy presence. Rights groups have questioned whether authorities are complicit with extremists or just incompetent.
Amnesty International repeated calls for Pakistan to do more to protect Hazaras, describing the failure to bring those responsible to justice as "shocking".
Attacks targeting Shiites in Pakistan have claimed almost 200 lives already this year. Human Rights Watch said more than 400 were killed in 2012, the deadliest on record for Shiites.
Hazaras have suffered disproportionately in Baluchistan, where authorities are also battling to suppress a separatist insurgency.