The wounded included six paramilitary soldiers and policemen who had arrived to investigate a smaller blast caused by a small grenade, moments earlier on Thursday.
Four cars had been set on fire by the blast outside the Holy Trinity Cathedral in the heart of the city. Several other cars, including two police vehicles, were damaged.
Hospital sources said none of those wounded had life-threatening injuries.
Tariq Jamil, a deputy inspector general of police, said the bomb in the car exploded soon after a firecracker was thrown from a passing car.
"We don't know whether it was a time device or a remote control device," he told reporters. "Obviously, all bomb blasts and other such acts are terrorist activities."
Since Pakistan joined the Washington's so-called "war on terror" in 2001, Karachi and other cities have seen a string of attacks on Western and Christian targets.
The blast took place just outside the office of the Pakistan Bible Society (PBS) adjoining the cathedral.
"I don't want to jump to conclusions, but we will begin our investigations by assuming the target was the office of
"I don't want to jump to conclusions, but we will begin our investigations by assuming the target was the office of Pakistan Bible Society"
Asad Ashraf Malick,
Pakistan Bible Society," said Karachi police chief Asad Ashraf
Among those hurt was the head of the PBS, Peter Pervez. Salim Khurshid Khokar of the All Pakistan Minorities Alliance said it was clearly an attack targeting Christians.
Pervez, who had a shoulder wound, but was not seriously
hurt, said he saw a helmeted man riding on the back of
a motorbike throw the device that caused the initial explosion.
A bomb disposal expert at the scene estimated the size of
the car bomb at eight to nine kilogrammes.
In October 2001, 15 members of the Anglican Church of
Pakistan were killed when gunmen burst into a church during a Sunday service in the town of Bahawalpur.