People had gathered for afternoon prayers at the mosque, which was also being used as a voter registration centre.
Some of the injured are said to be in a critical condition.
No group has claimed responsibility but the Islamic State group has carried out similar attacks in the past.
There have been a number of attacks on voter registration centres since the process started last month for October's parliamentary elections.
On 22 April, a suicide bomb attack at a voter registration centre in the capital Kabul killed at least 57 people.
The Kabul attack was claimed by IS, but the Taliban has also warned people not to take part in the elections, which are said to be a key test of President Ashraf Ghani's credibility.
More about: blast
