The device went off at about 7:30 am (local time) at the gate of the Federal High Court in the southern city, which is capital of Rivers state.
"The explosion damaged some parts of a car parked near the court gate. No harm or injury was recorded," said eyewitness Alice Jumbo.
Security personnel put out a fire quickly and secured the area, she added. Another witness, Harry Obele, said the blast sent people running for cover into the court.
Sporadic violence was a feature of the build-up to presidential and state elections in Rivers in March and April.
The APC, which previously controlled the state, complained of intimidation and strongarm tactics on the part of the PDP, as well as widespread rigging at the vote.