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.
The first hit the education ministry and then the second went off as medical teams arrived to deal with the aftermath, the Reuters news agency reports.
A lorry exploded at the same junction almost exactly five years ago, leaving more than 500 people dead - the worst such attack in the country's history.
After Saturday's attack, hundreds of people have gathered near the site, looking for missing family members.
Among those killed were a prominent journalist and senior police officer.
"I am here to tell the Somali people that such October attacks will not happen again, God willing," President Mohamud said after visiting the scene of the attack.
"The bombings were a message sent by the militants to show that they are still alive, despite the fact that they were defeated in battlefield by government forces," he added.
The African Union (AU) mission in Somalia said that the "attacks underline the urgency and critical importance of the ongoing military offensive to further degrade al-Shabab".
The US, Turkey, Qatar and Germany have all condemned the attack.
Al-Shabab has been battling the AU-backed federal government for control of Somalia for around 15 years.
The group controls much of southern and central Somalia, but has also been able to extend its influence into areas controlled by the government based in Mogadishu.