Reports indicate it was caused by a suicide bomber.
A heavy blast tore through the inside of Mogadishu government offices on Wednesday, leaving casualties in the building where the city's mayor is based, a security official said.
"The blast occurred inside but we are not sure what exactly caused it, some reports we are getting indicate it was caused by a suicide bomber... and there are casualties," said security official Mahdi Abdirahman.
He did not give further details on the wounded.
"The blast was very heavy, and I saw people, fleeing some with shrapnel wounds outside the Banadir administration headquarters," said witness Mohamud Shariif, referring to the regional government offices.
Mogadishu is regularly hit by attacks by the Al-Qaeda linked Al-Shabaab Islamist group, which has fought for more than a decade to topple the Somali government.
The city was on Monday struck by a car bomb which left 17 dead and more than two dozen wounded.