The Islamic State terror group, also known as ISIS or ISIL, has claimed responsibility for the deadly suicide bomb blast inside the mosque in Quetta's Satellite Town area.
The terror group, in the claim posted on the IS Pakistan Telegram Channel and in messages to some foreign wire services, said it has carried out the attack targeting some Afghan Taliban member.
Taliban spokesman, Qari Muhammad Yousuf, denied that any Afghan Taliban member was present inside the mosque.
The suicide blast -- which took place at a time when about 60 people were offering sunset (Maghrib) prayers -- killed 16 people and left 19 others injured, Balochistan government spokesperson Liaquat Shahwani said in a late night statement.
The deadly blast came three days after a bomb explosion claimed two lives in Quetta.
Reacting to the incident, President Arif Alvi and Prime Minister Imran Khan condemned the blast and expressed grief over the loss of lives and prayed for the peace of the departed souls and early recovery of those injured.
Prime Minister Khan demanded a report.
"I have demanded an immediate report on the condemnable cowardly terrorist attack in Quetta targeting a mosque & people at prayers. Have asked prov govt to ensure all medical facilities are provided to the injured. Martyred DSP Haji Amanullah was a brave & exemplary officer," he said on Twitter.
Khan said the best possible treatment would be given to the injured.
The nature of the explosion, which occurred inside the mosque during Maghrib prayers in Ghousabad neighbourhood, was not known.
Deputy Superintendent of Police (DSP) Amanullah was among the 16 people killed in the incident, Quetta Deputy Inspector General (DIG) Abdul Razzaq Cheema said.
According to some media reports, the slain police officer was the likely target.
Last month, unidentified gunmen killed the DSP's son in Quetta, The Express Tribune reported. Twenty others were also injured in the blast, the report said.
The mosque, which is located in a densely-populated Pashtun-majority area, was being searched by the bomb disposal squad and security personnel.
TV footage showed debris and shattered glass spread on the floor of the mosque.
Pakistan military's media wing ISPR said that troops of the Frontier Corp (FC) Balochistan reached the site and carried out joint search operation with the police.
"Every possible assistance be given to police & civil administration. Those who targeted innocents in a mosque can never be true Muslim," the ISPR quoted army chief General Qamar Bajwa as saying.
Balochistan Chief Minister Jam Kamal Khan condemned the incident and expressed grief at the loss of lives.
Reacting to the incident, Balochistan Home Minister Zia Langove condemned it, saying "terrorists were scared of Pakistan's development".
"Internal and external enemies are making failed efforts to create panic and unrest in the country," he said in a statement.
He said that "defeated terrorists will never be allowed to succeed".
Talking about the casualties in the incident, Langove said the death toll might rise as condition of some of the injured was critical after they were shifted to the Civil Hospital for treatment.
The blast occurred three days after two men were killed and over a dozen injured in a blast near a vehicle of the security forces in Quetta.
In May last year, a bomb blast at a mosque in the provincial capital Quetta killed two people, including a prayer leader and injured 28 others.
In August, an explosion took place inside a mosque during Friday prayers in the city.