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 said.
Officials at Abbasi Shaheed Hospital said they had received more than 30 injured people and three of them were in a serious condition.
Police officials told the media the bomb was planted in a motorcycle parked near a maternity home.
Three buses and several 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.
Noted cleric Tahir-ul Qadri, the leader of the Tehreek Minhaj-ul Quran, had also visited the MQM headquarters today to formally seek party's support and participation in a big rally planned on January 14 to march on Islamabad against a corrupt government and electoral system.
Yesterday, the MQM coordination committee had announced its support and participation in the rally.
The outlawed Tehreek-e-Taliban have said they would target MQM leaders and supporters after Altaf Hussain urged the government and people to unite and wage a war against the militants following the attack on child activist Malala Yousufzai.
Television channels reported that the blast which was heard in nearby areas had damaged shops and buses which were waiting to transport people who attended the rally.
Last week, a bomb blast on a passenger bus in the busy Cantt area had left six people dead and around 50 injured.
Karachi: A powerful bomb went off at a busy intersection in the southern Pakistani city of Karachi Tuesday, injuring over 50 people and kilig 4, witnesses and police officials said.
First Published: Tuesday, January 01, 2013, 20:12