According to the News 1st media portal, a restaurant employee alerted the police about the suspicious package.
Sri Lanka's defence minister confirmed that the blast was a controlled explosion, carried out by a special task force.
No explosive were found in the motorbike, according to the police statement.
Later in the day, Sri Lankan security forces carried out a third controlled explosion, the portal Ada Derana reported.
A suspicious motorcycle left unattended in the Pettah neighborhood was blown up by the specialists, according to the portal.
Eight explosions took place in Sri Lanka on Easter Sunday; three churches and three hotels were targeted. The number of people killed in a series of blasts that hit Sri Lanka has risen to 359 according to latest reports.
A massive security operation was launched following the attacks. Sri Lanka's authorities introduced and re-introduced curfews, deploying an extra 1,000 troops in the capital Colombo.
These were the worst attacks the country has faced since the end its 25-year-old civil war.