Pakistan navy spokesman said four navy officials aboard the bus were killed. A hospital official said a passerby was also killed.
"The bus was taking navy officers to a dockyard when a planted bomb exploded nearby," senior police official said. A civilian car and motorcycle were also damaged.
The bomb was placed in a gutter near the petrol pump on Shahra-e-Faisal near Karsaz Bridge and exploded when the bus arrived around 08:15 am.
About three kilograms of explosives were packed into the bomb which was detonated remotely, senior police official Iftikhar Tarar said.
He said the blast appeared to be the work of the same group behind twin bombings on Tuesday that killed four navy officials and wounded nearly 60 people in the worst attack on military officials in Karachi in years.
On Tuesday morning, two buses of the Pakistan Navy were also targeted when two bombs exploded near them. Four personnel of the Pakistan Navy, including a lady doctor and a junior officer, were killed while around 56 others sustained injuries.
