No casualties have been reported so far.
Police have stated that they are investigating the explosion, according to the Sri Lankan media outlet News First.
According to another news outlet, Ada Derana, the blast took place in a garbage can near the city court building. The police have no reason to believe that the new incident may be linked to the deadly Easter Sunday bombings.
The attacks, which hit churches and luxury hotels across Sri Lanka, are the worst the country has faced since the end of its 26-year civil war in 2009. On Tuesday, Daesh* reportedly claimed responsibility for the attacks.
Sri Lanka launched a criminal investigation immediately after the attacks, and has already arrested over 100 suspects.
On Tuesday, Sri Lankan President Maithripala Sirisena asked Defence Secretary Hemasiri Fernando and Inspector General of Police (IGP) Pujith Jayasundara to resign.
*Daesh (also known as ISIS/ISIL/IS) is a terrorist group banned in Russia