Twin car bomb explosions near a busy junction in Somalia's capital, Mogadishu, killed at least 100 people, President Hassan Sheikh Mohamud says.
Among the victims "who were massacred [were] mothers with their children in their arms", the AFP news agency quotes the president as saying.
He appealed for international medical help to deal with the 300 injured.
The president blamed the al-Shabab militant group for Saturday's attack which targeted the education ministry.
The pro-jihadist Somali Memo website has reported that the group has said it was behind the blasts.
An affiliate of al-Qaeda, al-Shabab has engaged in a long-running conflict with the federal Somali government.
President Mohamud, in power for five months pledged "total war" against the Islamist militants after they attacked a popular hotel in Mogadishu in August killing at least 21 people.
Saturday's blasts happened within minutes of each other, destroying buildings and vehicles in the vicinity.