A large explosion has hit the centre of the Kenyan capital, Nairobi, at a shopping complex on Moi Avenue. A BBC reporter says 28 people have been taken to hospital with injuries. The cause of the blast remains unclear. Initial reports suggested an electrical fault might be to blame, but Prime Minister Raila Odinga has said the blast had a terrorist link. There have also been reports of a man dropping off a bag at the site of the blast, minutes before the explosion. There have been several grenade and bomb blasts in Kenya since it sent troops into Somalia last year to tackle the al-Shabab Islamist militant group. Al-Shabab has repeatedly threatened to stage revenge attacks against Kenya. Mr Odinga told reporters at the site of the blast: "This is terrorism... this is a heinous act, we are under threat, but we will not be cowed." 'No power fault' Police commissioner Mathew Iteere had earlier said: "We are able to prove it was not a grenade attack or bomb attack... from the preliminary conclusions we do think it was an electrical fault," he said. But Kenya Power, the country's sole electricity distributor, says initial investigations have ruled out any electrical malfunction. In a statement, the company said: "A rapid response technical team from Kenya Power visited the premises upon learning of the incident and found out the following: *The affected building has no ground mounted transformer inside it or outside that would explode *The team found all the electrical connections to the building including the cutouts (fuses) on the Kenya Power side that would otherwise blow in the event of a short circuit inside the building intact *On the Kenya Power network that supplies the general area, no protective devices operated. Protective devices would switch off power network to prevent a fault in a premises, such as the affected building from coming back into the rest of the network." It concluded that it was "therefore not possible that the explosion was caused by an electricity fault". The BBC's Kevin Mwachiro says glass and shoes and clothes from the small shops inside the building have been scattered across the street. TV pictures have shown people pouring into the streets from nearby buildings to get away from the scene of the fire, the AP news agency reports. Moi Avenue is a major road which would have been busy during the lunch hour, AP says. According to an eyewitness, there was a huge blast and debris flew in different directions injuring people in the vicinity, the Nation newspaper reports. The powerful explosion shook buildings in the surrounding area and the evacuation process began, the Nation says.