MOGADISHU, Somalia — Car bombs, grenade attacks, assassinations and abductions by Shabab insurgents shatter the fragile veneer of calm in Somalia with such regularity that barely a week goes by without a deadly assault.

Two large explosions occur on average in Mogadishu, the capital, every month. In June, more than 30 people were killed when militants stormed a popular restaurant in the city. Politicians and businessmen have been shot dead in broad daylight. Aid workers have been kidnapped, scores of civilians gunned down or killed in roadside bombs. The violence is so pervasive that embassies are inside the international airport.

But even by Somalia’s standards, the twin truck bombings this past Saturday in Mogadishu that killed more than 270 people, including at least three Somali-Americans, were unusual in their scale and brutality. Although the Shabab, Somalia’s Islamist extremist organization, has not publicly claimed responsibility, its members are thought to have orchestrated the attack, one of the country’s most destructive, and may have even received inside help.

Some Somalis are referring to the attack as their 9/11, and the president has declared three days of mourning. Even a reformed Shabab leader was seen donating blood to victims, calling on his former colleagues, some of whom have proclaimed allegiance to Al Qaeda while others support the Islamic State, to renounce violence as he had done. “There is no other solution than to unite and fight against the people” behind the attacks, he told reporters.