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Ashfaq Ahmad, from the Edhi ambulance rescue service, said its ambulances had taken four injured people from the blast site near an office of the paramilitary Rangers.
Deputy Inspector General of the Karachi police, Javed Odho, said the bomb appeared to be targeting residences for the Rangers. He had no information about injuries or whether anyone was killed.
Witnesses reported seeing a large plume of smoke in the sky.
Karachi in southern Pakistan and is the country's largest city.
There was no claim of responsibility but Taliban militants are known to operate inside the city.
