At least three people were killed and 19 were wounded in a blast inside Rehmania mosque in Quetta's Pashtoonabad area during Friday prayers, according to Civil Hospital medical superintendent (MS) Dr Saleem Abro.
The injured were shifted to Civil Hospital Quetta.
Deputy Inspector General (DIG) Quetta Abdul Razzaq Cheema confirmed the casualties and said that the attack was carried out through an improvised explosive device (IED). According to Cheema, the blast occurred before Friday prayers started.
There has been no claim of responsibility as yet.
The DIG said that out of 618 mosques, 100 have been provided with security. He added that 1,500 police officials have been posted across the city.
Balochistan Chief Minister Jam Kamal Khan Alyani condemned the blast and summoned a report on the incident. He expressed grief over the deaths and multiple injuries caused by the blast.
"Those who make innocent people the target of terrorism in this blessed month and on this blessed day deserve a severe punishment," he said, while ordering security measures in the city to be further strengthened.
Alyani condoled with the family of the deceased and prayed for the speedy recovery of the injured.
In a post shared on Twitter, State Minister for States and Frontier Regions Shehryar Afridi condemned the attack, saying: "Another nefarious act by evil forces in Quetta."
Even in Ramazan they choose to target civilians praying in a masjid, Afridi wrote, adding: "It is so painful [and] astonishing to see that evil forces are targeting innocent civilians."
National Assembly Speaker Asad Qaiser and Deputy Speaker Qasim Suri also strongly condemned the attack and said that those who attack mosques and other places of worship are enemies of humanity and have no relation to any religion.
They ordered law enforcement agencies to bring those who were involved in the incident to justice while expressing grief over the deaths caused by the blast and praying for the speedy recovery of the injured.
Leader of the Opposition in the National Assembly Shahbaz Sharif also condemned the attack.
"The person who attacks the house of God on a Friday in the month of Ramazan cannot be a Muslim," he said, adding that the continuous rise in terrorist incidents was extremely worrisome.
PPP co-Chairman Asif Ali Zardari also issued a condemnation and stressed the need for "eliminating the nurseries that nurture extremist ideologies".
"Those who spill the blood of humans are unforgivable," he said and urged the government to provide protection to the people. He extended his sympathies to the families of the victims.
This is the fifth act of violence in Balochistan since the beginning of Ramazan.
On May 11, at least five people lost their lives in a terror attack targeting the Pearl Continental in Gwadar, the army's media wing said in a statement, as it announced the conclusion of its clearance operation of the hotel.
Three days later, on May 14, four policemen were martyred in a blast near a mosque in the Satellite Town area of Quetta.
Prior to the start of the Holy month, 20 people were killed and 48 others injured in the Hazarganji market blast on April 12, targeting Quetta's Hazara community. The same day, two people were killed in a blast targeting a Frontier Corps vehicle in Chaman.
Less than a week later, 14 passengers were offloaded from a passenger bus on the Makran Coastal Highway and shot dead by unidentified assailants.
Additional reporting by Javed Hussain and Nadir Guramani