The blast occurred under a fly-over at Sharah-e-Faisal in Karachi after 9:15 pm (local time), witnesses said.
No one was present at the spot at the time of the explosion, police officials said.
Earlier in the day, Shia political and student groups had organised a demonstration at the same spot to protest the killing of nearly 90 people in a bomb attack in a Shia-dominated area of Quetta city on Saturday.
The protest ended at about 7 pm (local time).
"The blast was caused by a remote controlled device and it caused a deep crater," senior police official Umar Khatab said.
He said it was too early to say if the bomb was planted because of the sit-in by the Shias.
Police did not report any immediate casualties but the Shahrah-e-Faisal road was closed for traffic as the bomb disposal squad searched for more explosive devices.
Police said the bomb would have caused considerable damage if it had gone off earlier.
Footage on television showed the blast had created a crater several feet wide.
Authorities announced that schools in Karachi would remain closed tomorrow in view of tensions in the city.
No group claimed responsibility for the blast.
Karachi: A powerful bomb went off at a spot in the southern Pakistani port city of Karachi where Shia groups had staged a protest on Monday though there were no casualties, police officials said.
First Published: Monday, February 18, 2013, 23:28