The blast took place at Hub River Road near Naval Colony in Baldia Town.
Sources said the bomb was planted in a motorcycle.
The four victims who were injured include a retired DSP. All four of them were shifted to a hospital for medical treatment.
Police surrounded the site and investigations into the blast are underway.
Online adds
Rangers arrested 53 suspected criminals with arms during special operations and snap-checking in Karachi.
Spokesman Pakistan Rangers the media that they recovered two kalashnikovs, 32 pistols and over 400 rounds from the custody of the criminals.
He also added that all detainees found with weapons have been handed over to the police stations according to their jurisdictions and investigations will be conducted by them.
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