A powerful bomb went off on a bus in the centre of the Sri Lankan capital wounding four people Tuesday, police and medical officials said. The bus had been evacuated shortly before the explosion because passengers had spotted a suspicious parcel, military spokesman Brigadier Udaya Nanayakkara said. Three of those hurt were only suffering from ear problems caused by the blast, national hospital director Hector Weerasinghe said. The force of the explosion shook buildings in the city centre, where security is tight because of fears of attacks by Tamil Tiger rebels.