The death toll from the single deadliest attack in Somalia's history rose to 276 Sunday as emergency workers feverishly dug through the rubble of a Mogadishu bomb blast that collapsed buildings and set nearby cars ablaze.

About 300 people were injured when the truck explosion rocked a crowded shopping district Saturday. On Sunday, Mayor Tabid Abdi Mohamed issued a plea for residents and businesses that owned earth-moving equipment to bring it to the blast site to help the desperate search for survivors — and bodies.

President Mohamed Abdullahi Mohamed, known as Farmajo, called for three days of mourning.

"Terror won't win," he said in the capital after donating blood and visiting some of the scores wounded. "I call on our citizens to come out, extend help, donate blood and comfort the bereaved. Let’s get through this together."

Abshir Abdi Ahmed, the deputy speaker of Somalia's upper house of parliament, earlier said the death toll could still rise.

Prime Minister Hassan Ali Khaire blamed the attack on the Somali militant group al-Shabab, which has not commented on the explosion. The group has carried out a series of attacks in recent years aimed at establishing a radical Islamist state, and Farmajo had stepped up efforts to combat the militants.

“They don’t care about the lives of Somali people, mothers, fathers and children,” Ali Khaire said. “They have targeted the most populated area in Mogadishu, killing only civilians.”

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Somalia's SONNA news agency said the attack apparently involved two bombs, although details remained unclear. The blast took place in the popular Hodan district, busy with shops, businesses and hotels in northwestern Mogadishu.

Residents were stunned by the enormity of the attack.

"In our 10-year experience as the first responder in #Mogadishu, we haven't seen anything like this," the Aamin Ambulance service tweeted Sunday.

The Mogadishu bombing is one of the deadliest attacks in sub-Saharan Africa, larger than the Garissa University attack in Kenya in 2015 and the U.S. Embassy bombings in Kenya and Tanzania in 1998.

The National Union of Somali Journalists said a freelance cameraman was among the victims in Saturday's attack, and four other journalists were wounded. Qatar said its embassy in Somalia was severely damaged and one staffer was slightly injured.

Relatives of the dead and wounded rushed to hospitals overwhelmed with victims.

“There’s nothing I can say," Zainab Sharif, a mother of four who lost her husband, said outside the hospital where he was pronounced dead. "We have lost everything."

The U.S. State Department condemned the "senseless and cowardly act." The U.S. mission in Somalia lauded "the heroic response of the Somali security forces and first responders and Somali citizens who rushed to the aid of their brothers and sisters."

The United Nations expressed outrage at the attack and condolences to families of the victims. Michael Keating, the U.N. special envoy to Somalia, tweeted Sunday, "Solidarity among Mog citizens is inspiring, including 2 donate blood in wake of yesterday’s horrendous attack that killed so many civilians."

Al-Shabab, which translates as the Youth, emerged from a now-defunct Islamic group that ruled Mogadishu a decade ago until Somali forces liberated the city. Al-Shabab has links to al-Qaeda and other terrorist groups and has drawn fighters from neighboring countries.

Al-Shabab has also staged attacks over the border in Kenya, the most deadly being the one at Garissa University, when gunmen stormed the school in a bloodbath that killed 148 and wounded dozens more.

The militant group promotes strict sharia law, which includes amputating the hands of thieves. The group's attacks have added to the difficulties faced by Farmajo, who was elected in February to lead the country of 14 million people that ranks among the poorest nations in the world.

Farmajo, 54, is the first democratically elected president of Somalia in more than two decades. The election was conducted by members of parliament instead of voters due to threats of violence by al-Shabab. Lawmakers held their vote in an aircraft hangar for added security.

Farmajo has close ties to the United States, holding dual Somali-U.S. citizenship and a degree from the State University of New York-Buffalo.

Contributing: The Associated Press