QUETTA, Pakistan - At least 73 people were killed and over 160 others injured when a suicide bomb blast hit Pakistan's southwest city of Quetta on Friday, local Urdu TV channel Dunnya quoted police sources as saying.

Meanwhile, a local organization called Shiite Conference, Balochistan announced a 40-day mourning in the city as most of the victims in the terrorist attack were Shiite Muslims, a minority sect whose population accounts for about five to six percent of the country's total.

Volunteers help injured men to an ambulance from the site of a suicide attack during a Shi'ite procession in Quetta September 3, 2010. [Photo/Agencies]

Shortly after the blast, police have cordoned off the area and fired in the air to hold back people who tried to enter the blast site to search for their relatives.

A violent protest rampaged in the wake of the blast, said Xinhua reporters, adding that several shops were torched and fierce gunshots were heard across the city.

Pakistani prime minister Syed Yousuf Gilani strongly condemned the attack and ordered an immediate investigation into the incident.

A banned Sunni radical group called Lashkar-e-Jhangvi later claimed responsibility for the attack. The Punjab-based terrorist group founded in 1996 was also claimed carrying out at least two suicide bomb attacks on a Shiite Muslim mourning procession in Lahore, capital of the eastern Pakistani province of Punjab, on Wednesday night, killing 37 people and injuring over 200 others.

On Thursday night, a spokesman of Lashkar-e-Jhangvi said that the attacks were in revenge for the killing of their leaders by Shiite Muslims and threatened more attacks against Shiites.

Friday's suicide blast in Quetta was the most serious one of its kind since the end-July in Pakistan which was also devastated by the worst flood in living memory.

Injured of a suicide attack wait for treatment after they brought at civil hospital in Quetta September 3, 2010. [Photo/Agencies]