The blast occurred at Ayesha Manzil intersection, located a short distance from Jinnah Ground where the Muttahida Qaumi Movement had organised a rally this afternoon.
People leaving the rally were hit by the blast at about 7.15 pm, witnesses and police officials said.
Four persons were killed by the blast and five others were critical in hospital, said Sagheer Ahmed, the Health Minister of Sindh province.
DIG Asif Ejaz Sheikh told the media that 50 injured people were admitted to several nearby hospitals.
Officials at Abbasi Shaheed Hospital said three of the injured were in a serious condition. Women and children were among the injured. The bomb was planted in a motorcycle parked near a maternity home.
Many persons were hit by ball bearings packed into the bomb, which was triggered by remote control, DIG Sheikh said.
Three buses and five vehicles standing near the motorcycle were damaged by the blast, which was heard from several kilometres away.
The explosion also damaged nearby buildings and shops.
Police cordoned off the area and launched a search operation. Rescue operations were hampered by a traffic snarl and the large number of people who gathered in the area.
The blast caused panic and fear in the city and led to massive traffic jams and bottlenecks with shops and petrol pumps closing down.
Though MQM leaders have been killed in target killings in the past, this is the first time that terrorists have directly targeted the MQM in a bomb attack.
MQM, which is the single largest party in Pakistan's biggest city, has its headquarters close to Aisha Manzil and the Jinnah Park is used regularly for its rallies.
The blast took place just after MQM chief Altaf Hussain had finished addressing the rally via telephone from London.
