QUETTA, Pakistan: Pakistani forces on Tuesday killed four men and arrested seven others accused of killing Shiite Muslims, including an alleged mastermind of a bomb attack that killed 89 people, officials said.

The operation was carried out on the outskirts of the southwestern city of Quetta, where thousands of Shiites are demanding army protection and refusing to bury the victims of Saturday's bomb attack on their ethnic Hazara community.

"Those who were killed were high-profile target killers," said the home secretary of Baluchistan province, Akbar Hussain Durrani, adding that one of the masterminds of Saturday's attack was among those in custody.

It was not immediately clear whether the raid would be enough to persuade Shiite leaders to call off protests, which have spread to Pakistan's largest city Karachi and forced the closure of the main road to the airport in Islamabad.

The minority community is furious at the authorities' failure to tackle record violence against Shiites, who make up 20 percent of Pakistan's predominantly Sunni Muslim population of 180 million, and prosecute those responsible.

Pakistan is due to hold a general election by mid-May and the sectarian violence, plus a Taliban insurgency in the northwest, has raised questions about the conditions under which polls will be held.

The arrests came just hours before Pakistani cabinet ministers were due to arrive in Quetta to try to negotiate an end to the protest, which has seen thousands of men, women and children camp out since late Sunday.

A similar protest after 92 people were killed by suicide bombers at a Hazara snooker hall in Quetta on January 10 only ended after four days when Islamabad sacked the provincial government and imposed governor's rule.

The banned militant group Lashkar-e-Jhangvi claimed both attacks in Quetta, a small town where the military and intelligence agencies have a heavy presence and where rights groups have questioned whether authorities are complicit with extremists or incompetent.

Daud Agha, chairman of the Baluchistan Shia Conference, one of the groups taking part in the protest, expressed satisfaction over the raid and announced that a time would be announced for the burials to begin.

But the Shiite Wahdatul Muslemeen party said the vigil would continue.

Violence between Sunni and Shiite Muslims last year reached record levels with ethnic Hazaras suffering disproportionately in Baluchistan, where authorities are also battling to put down a separatist Baloch insurgency.

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.

Amnesty International repeated calls for Pakistan to do more to protect Hazaras, describing the failure of the authorities to bring those responsible to justice as "shocking".

"The failure to bring these perpetrators to justice sends the signal that they can continue to commit these outrageous abuses with impunity," said Isabelle Arradon, Amnesty International's Deputy Asia-Pacific Director.

The campaign group said at least half of all Shiite deaths were suffered by Hazaras, one of the smallest communities in Pakistan.

"In fact, the attacks in January and February constitute some of the worst killings in Pakistan's recent history," Arradon said.

Hundreds of Shiite protesters camped out overnight between the capital and its twin city Rawalpindi, forcing authorities to close the main road to the airport and causing huge traffic snarl-ups.

In Karachi, Pakistan's largest city, hundreds of Shiite protesters camped out in at least 10 separate locations. Traffic remained thin, schools and most shopping areas were closed for a second day.

Security officials said at least five people were shot dead overnight, three Sunnis and a Shiite on sectarian grounds.