According to a local official, Alam Baloch, a senior police officer and a cleric leading the prayers were among the dead in the bombing in the city of Quetta, the headquarters of the volatile province of Balochistan.
Baloch explained that the bomb exploded when dozens of people were leaving the hall after the evening prayers.
Regional police Chief Abdul-Razzaq Cheema said initial evidence suggested a suicide bomber might have blown himself up.
Cheema added that at least 13 injured people were taken to a hospital in the city and were being treated.
According to him, no group has claimed responsibility for the attack and it was also not known if either the police officer or the cleric were the target.
Islamist militants from both the Taliban and the extremist Islamic State militia have attacked both security officials and clerics in the past in Quetta.
The province of Balochistan, which borders with Iran and Afghanistan, has been under attack by Islamist militants, sectarian extremists and nationalist rebels seeking the liberation of the land from Pakistan.
However, about 70,000 people have lost their lives to terrorist attacks since the country joined the global war against groups like al-Qaeda and the Taliban in neighbouring Afghanistan.