At least 76 people have been killed by a vehicle bomb detonated during morning rush hour in Somalia's capital.
The blast took place at a checkpoint at a busy intersection in Mogadishu. More than 90 people were also injured.
No group has yet claimed responsibility for the bomb but al-Shabab militants have often carried out attacks there.
President Mohammed Abdullahi Farmaajo said its aim was to demoralise the Somali people, but that jihadists would never stop the country rebuilding.
Al-Shabab - a group of Islamist militants, allied to Al-Qaeda - has waged an insurgency for more than 10 years. It was forced out of the capital in 2011 but still controls areas of the country.
Witnesses described carnage at the scene.
"All I could see was scattered dead bodies ... amid the blast and some of them burned beyond recognition," said Sakariye Abdukadir, who was close to the blast.
"May Allah have mercy on the victims of this barbaric attack," the former internal security minister added.
At least 16 of the victims are students from Banadir University who were travelling on a bus that was struck by the vehicle bomb, AFP report.
"This was a black day," university chairman Mohamed Mohamud Hassan said, according to AFP. "It was a day when parents who have sent their children to learn were sent back the dead bodies of their children."
An ominous warning ahead of a big year
Analysis by Andrew Harding, Africa correspondent
This attack is another stark reminder of how difficult it is to protect a city as large as Mogadishu against a determined militant group - al-Shabab - which has no qualms about attacking overtly civilian targets. It will also underline and fuel longstanding concerns that al-Shabab has infiltrated elements within the Somali state and its security forces, enabling it to keep conducting this kind of operation in the capital.
Despite losing territory to African Union (AU) and Somali government forces in recent years, al-Shabab has proved to be a remarkably resilient organisation. It has been able to exploit the fragility of Somalia's fledgling government institutions and the centrifugal regional and clan interests, which continue to undermine the country's security and unity.
Today's bloodshed carries with it an ominous warning for the year ahead, which is due to see Somalia hold crucial, but potentially destabilising, one-person one-vote elections for the first time in decades. In addition, there is enduring concern about the fate of Amisom - the 20,000-strong AU army that plays such a central role in the country's security, and which is in theory scheduled to begin winding down, to be replaced by Somalia's new national army.
Three witnesses told Reuters the blast had taken place next to a small group of Turkish engineers who were constructing a road.
Somali Foreign Minister Ahmed Awad tweeted that two of the Turkish engineers died.
Many of the dead were "students with ambition, and hardworking men and women", he wrote.
Turkey has been a major donor to Somalia since a famine in 2011.
President Mohamed Abdullahi Farmaajo condemned the attack.
"This enemy works to implement the destructive will of international terrorism, they have never done anything positive for this our country, they have not constructed a road, never built hospitals and not education centres as well," he was quoted as saying by the Somalia national news agency Sonna.
"All they do is destruction and killing and the Somali public are well informed about this."
Five people were killed earlier this month when al-Shabab attacked a Mogadishu hotel popular with politicians, diplomats and military officers.