According to the Dawn newspaper, the explosion occurred near the truck that was part of the rally near Bait-ul-Mukarram Masjid.
A majority of the wounded received "very minor injuries", while some were discharged, a local official said.
The Sindhudesh Revolutionary Army claimed responsibility for the attack through social media, the news outlet read.
The rally was held by the Jamaat-i-Islami* organization in connection with the first anniversary of India's abolition of the state of Jammu and Kashmir, whose territory is claimed by Pakistan.
* Jamaat-i-Islami is a terrorist organization banned in Russia