Karachi: Four parcel bombs exploded in Pakistan's violence-plagued commercial capital Karachi on October 16, injuring at least nine people, police said.

The first three blasts occurred at two police stations and a government building in the space of 30 minutes within a two kilometre radius in the heart of Pakistan's largest city.

"All three were parcel blasts. They went off as the parcels were opened," a senior police officer told AFP.

A fourth parcel bomb was later discovered near one of the targetted police stations just before 15:30 hours, local time (10:30 GMT). Police said they defused the bomb.

There were no casualties. Just before 13:30 hours, local time (08:30 GMT) a parcel bomb exploded in the office of the Pakistani police force's top al-Qaida investigators at the Crime Investigation Department (CID), CID investigator Mukhtar Junejo told AFP.

"We received two parcels and when an officer opened one of the parcels it exploded and two police officers received minor injuries," Junejo said.

The CID was set up in Karachi 12 months ago to track down fugitives from Osama bin Laden's network as well as Pakistani Islamic militants.

Another parcel bomb went off in the Karachi police force's operational headquarters in a district called Artillery Medan, injuring four people, police officer Abdul Rauf told AFP.

A third parcel bomb exploded in the offices of the Sindh provincial government's Home Affairs Department at Tughlaq House, Karachi police chief Asad Jehangir told AFP.

Two people were injured in the blast in Tughlaq House.

"The explosions occurred within a 35 to 40 minute period," Karachi deputy police chief Tariq Jamil told AFP.

"It is an act of terrorism," Sindh home secretary retired Brigadier Mukhtar Sheikh declared.