A devastating explosion, claimed by the Islamic State, ripped through a Sufi shrine in Sehwan town of Pakistan's Sindh province on Thursday, killing at least 100 people, including 12 women and four children, and injuring over 200 others.

Assistant Superintendent of Police in Sehwan said a suicide bomber entered the Lal Shehbaz Qalandar shrine through its Golden Gate. The attacker blew himself up after throwing a grenade, which failed to explode, he said.

The explosion took place at the spot where the "Dhamaal" Sufi ritual was being performed within the premises of the shrine.

The Islamic State claimed responsibility for the attack, the group's affiliated news agency reported.

A stampede followed the blast as a large number of devotee, including women and children, were present in the shrine.

Contingents of police reached the shrine that is situated slightly off the Super Highway in Dadu district of Sindh. Hundreds of people gather at the Sufi saint's shrine every Thursday for religious rituals.

An emergency has been declared in all hospitals of the area, with the injured being shifted to Liaquat Medical Complex Jamshoro and the sub-district hospital.

Prime Minister Nawaz Sharif decried the attack: "The past few days have been hard, and my heart is with the victims," Sharif said.  

"But we can't let these events divide us, or scare us. We must stand united in this struggle for the Pakistani identity, and universal humanity."

Moments after the powerful bombing the Pakistan military issued a strongly worded statement to "hostile powers", saying it will respond to such attacks.

Director General Inter-Services Public Relations Maj Gen Asif Ghafoor said: "Recent terrorist acts are being executed on directions from hostile powers and from sanctuaries in Afghanistan. We shall defend and respond."

"Each drop of the nation's blood shall be revenged, and revenged immediately. No more restraint for anyone," the army chief was quoted as saying.

Chief Minister Murad Ali Shah said he had directed rescue teams to reach the spot of the attack.

The last attack on a Sufi shrine took place on November 12, 2016, when a suicide bomber struck the shrine of Shah Norani in Khuzdar district of Balochistan, killing at least 52 people and leaving 102 injured.

The explosion had taken place where the Dhamaal was being performed. At least 500 people were gathered at the spot to observe the ritual.

The blast was the latest in a series of attacks to hit the country this week.

A suicide bomber targeted a government office in the Mohmand tribal area on Wednesday, while a separate attack on the same day targeted government employees in Peshawar. Six people were killed in total.

On Tuesday, two police officers were killed while trying to defuse a bomb in Quetta.

Sixteen people were also killed in a suicide bombing at a protest rally in the Lahore on Monday.