Islamabad, Pakistan (CNN) -- A three-day mourning period began Friday in a southwestern Pakistani province where a series of bomb blasts killed nearly 100 people the previous day in the bustling city of Quetta.
The deadliest explosions were two suicide bombings in a predominantly Shiite neighborhood known as Alamdar Road.
One blast brought police, rescue workers and journalists rushing to the scene. It was swiftly followed by another explosion -- set off by a man sitting in a car with over 100 kilograms of explosives -- that caught many of those responding to the initial attack.
The double bombing, described by police as one of the worst attacks on the Shiite minority, killed 85 people and wounded about 150.
Lashkar-e-Jhangvi, a banned Sunni militant group, claimed responsibility for the Alamdar Road attacks.
Shiites, a minority sect in the mainly Sunni Muslim Pakistan, face persecution from extremists. Last month, more than 20 Shiite pilgrims were killed when a car bomb detonated near the buses they were traveling in.
Mir Zubair Mehmood, a Quetta police official, said the Alamdar attacks were motivated by Sunni and Shiite sectarian differences.
Another blast in Quetta on Thursday struck a security checkpoint in a busy market, authorities said. A bomb planted in a car detonated as security forces entered the area, killing 12 people and wounding 45, according to Wazir Khan Nasir, a police spokesman. Nasir blamed the attack on Baloch insurgents.
A fourth bomb went off by the side of the road leading to the city's airport, wounding three.
Quetta is the capital of Balochistan province.
Balochistan Chief Minister Nawab Raisani announced the mourning period for the province late Thursday in the aftermath of the bombings. Over the next three days, commercial activity in Quetta, one of Pakistan's largest cities, will be suspended.
Balochistan is regularly plagued by violence from myriad causes. As well as the sectarian attacks on Shiites, the unrest in the province is believed to be fomented by several insurgent groups, including the separatist Balochistan Liberation Army and the Pakistani Taliban.
Although Balochistan is the largest province in Pakistan geographically, analysts and some locals have criticized the federal government for neglecting it, leading to instability.
Prime Minister Raja Pervez Ashraf condemned the blasts in Quetta.
"The prime minister, while expressing his heartfelt condolences and sympathies with the bereaved families, reiterated the government's resolve to stamp out the menace of militancy and terrorism from the country in its all shapes and manifestations," said a statement released by his office.
Elsewhere on Thursday, a bomb went off in a preaching center on the outskirts of the city of Mingora in the Swat Valley, police said. The blast killed 21 people and wounded 70, according to Dr. Jamil Khan, a doctor at Mingora hospital. So far, no group has taken responsibility for the attack.
Mingora is in Khyber-Pakhtunkhwa province in Pakistan's northwest.
The U.S. ambassador to Pakistan, Richard Olson, expressed condolences to the families of those killed in the attacks.
"The United States stands with the people of Pakistan in strongly condemning these senseless and inhumane acts," he said in a statement released Friday.
CNN's Aliza Kassim in Atlanta and journalist Nasir Habib in Islamabad contributed to this report.