The explosives weighing around 4 kgs were planted on a motorcycle which was set off by remote control when a police van was passing at the busy Korangi intersection.
"One civilian was killed in the blast while seven others were injured," a senior police official said.
He said the obvious target of the blast was the rapid force police van which was passing through with a rickshaw behind it when the explosion took place.
Policemen have been frequently targeted in Karachi since a clean-up operation was launched by the government in the city against militants and criminals.
Around 150 policemen have been killed this year in targeted killings and bomb blasts.