MOGADISHU — Thousands of people took to the streets of Somalia’s capital Wednesday in a show of defiance after the country’s deadliest attack, as two people were arrested in connection with Saturday’s massive truck bombing that killed more than 300.

Wearing red headbands, the crowd of mostly young men and women marched through Mogadishu amid tight security. They were answering 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.

The crowd marched toward the site of the attack, which also wounded nearly 400. Scores remain missing.

“You can kill us, but not our spirit and desire for peace,” said high schoolteacher Zainab Muse. “May Allah punish those who massacred our people,” said university student Mohamed Salad.

It was not all peaceful. At least three people, including a pregnant woman, are injured after security forces opened fire while trying to disperse protesters marching toward the site of Saturday’s huge truck bombing in Mogadishu, said Capt. Mohammed Hussein. Both Somali police and African Union soldiers were at the scene and opened fire, he said.

Somalia’s government has blamed the attack on the Al-Shabaab extremist group, which often targets Mogadishu but has not commented. Analysts have suggested that Al-Shabaab, an ally of Al-Qaeda, may have avoided taking responsibility because it did not want to be blamed for the deaths of so many civilians.

According to a Somali intelligence official investigating the attack, an overloaded truck covered with a tarpaulin approached a security checkpoint outside Mogadishu early Saturday.

The truck, covered in dust, aroused suspicion from soldiers who ordered the driver to park and get out. The driver, a man who soldiers said behaved in a friendly manner, then made a phone call to someone in the capital.

The driver then passed the phone to the soldiers to speak to a well-known man who vouched for the truck and persuaded soldiers to allow it to proceed into the city, said the Somali intelligence official.

Once through the checkpoint, the truck started to speed along the sandy, potholed road and raced through another checkpoint where soldiers opened fire and flattened one of its tires.

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. The man who vouched for the truck has been arrested and is being held in jail, said the official.

The massive bomb, weighing between 600 kilograms and 800 kilograms, was meant for Mogadishu’s heavily fortified international airport, according to security officials. Several countries’ embassies are located there.

The driver probably decided to detonate on the street instead because several more checkpoints were ahead before the airport, the Somali intelligence official said.

“Another reason that he would not proceed further is the fact that security forces were coming after it,” the official said, speaking on condition of anonymity because he was not authorized to speak to reporters.

The truck bomber had an accomplice driving a smaller car, a Toyota Noah minivan packed with explosives that took another route, said a Somali intelligence official who spoke on condition of anonymity because he was not authorized to speak to the press. Security forces stopped the vehicle at a checkpoint near the airport, forcing the driver to park and get out.

As soldiers questioned the driver, the minivan detonated, the official said.

The minivan’s driver is currently in a prison in Mogadishu, said a senior Somali police officer, Capt. Mohamed Hussein. — AP