They said two people have been arrested.
Wearing red headbands, the crowd of mostly young men and women marched through Mogadishu amid tight security. They answered a call to unity by Mayor Thabit Abdi, who said "we must liberate this city which is awash with graves."
Some in Somalia have called the bombing their 9/11, while asking why one of the world's deadliest attacks in years hasn't drawn the kind of global attention given to extremist assaults elsewhere.
"You can kill us, but not our spirit and desire for peace," said high school teacher Zainab Muse. "May Allah punish those who massacred our people," said university student Mohamed Salad.
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It was not all peaceful. At least three people, including a pregnant woman, were injured after security forces opened fire while trying to disperse protesters marching toward the attack site, said police Capt. Mohammed Hussein. Both Somali police and African Union soldiers were at the scene and opened fire, he said.
Somalia's government has blamed Saturday's attack on the al-Shabab extremist group, which often targets Mogadishu but has not commented. Analysts have suggested that al-Shabab, an al-Qaida ally, may have avoided taking responsibility because it did not want to be blamed for the deaths of so many civilians.
The truck, covered in dust, aroused the suspicions of soldiers who ordered the driver to park and get out. The driver, a man who soldiers said behaved in a friendly manner, made a phone call to someone in the capital.
The driver passed the phone to the soldiers to speak to a well-known man who vouched for the truck and persuaded soldiers to allow it into the city, said the Somali intelligence official.
The driver continued before stopping on a busy street and detonating. The blast leveled nearly all nearby buildings in one of Mogadishu's most crowded areas.