The death toll from a powerful twin blast in Somalia's capital has risen to 276 with about 300 others injured, making it the deadliest attack ever in the Horn of Africa nation, officials said.

Key points: The two blasts hit areas densely populated with civilians

The two blasts hit areas densely populated with civilians The government has blamed the Al Shabaab extremist group, who has yet to comment

The government has blamed the Al Shabaab extremist group, who has yet to comment The attack marks one of the deadliest to ever hit sub-Saharan Africa

Information Minister Abdirahman Osman, who revealed the updated death toll, described the attack as "barbaric".

Mogadishu, a city long accustomed to deadly bombings by Al Shabaab, was stunned by the force of Saturday's blast.

The attacks shattered hopes of recovery in an impoverished country left fragile by decades of conflict, and it again raised doubts over the government's ability to secure the seaside city of more than 2 million people.

As angry protesters gathered near the scene of the attack, Somalia's government blamed the Al Qaida-linked Al Shabaab extremist group for what it called a "national disaster".

However, Africa's deadliest Islamic extremist group, which often targets high-profile areas of the capital, had yet to comment.

Loading

"They don't care about the lives of Somali people, mothers, fathers and children," Prime Minister Hassan Ali Khaire said.

"They have targeted the most populated area in Mogadishu, killing only civilians."

Saturday's twin attacks in Mogadishu mark one of the deadliest attacks ever in sub-Saharan Africa, larger than the Garissa University attack in Kenya in 2015 and the US Embassy bombings in Kenya and Tanzania in 1998.

The first explosion to hit was a huge truck bomb that ripped through the heart of the city's K5 Junction area — the K5 junction is home to government offices, hotels and restaurants.

The second bomb went off two hours later in the Medina district.

'This is really horrendous, unlike any other time'

Somalis gather and search for survivors by destroyed buildings. ( AP: Farah Abdi Warsameh )

Ambulance sirens echoed across the city a day after the blast as bewildered families wandered in the rubble of buildings, looking for missing relatives.

The death toll was initially given as 22 with officials fearing the toll would continue to climb.

Loading

Doctors at Mogadishu hospitals struggled to assist badly wounded victims, many burned beyond recognition.

"This is really horrendous, unlike any other time in the past," said Dr Mohamed Yusuf, the director of Medina hospital.

Nurse Samir Abdi described "unspeakable horrors".

"Nearly all of the wounded victims have serious wounds," she said.

President Mohamed Abdullahi Mohamed declared three days of mourning and joined thousands of people who responded to a desperate plea by hospitals to donate blood for the wounded victims.

"I am appealing all Somali people to come forward and donate [blood]," the President said.

Many victims died at hospitals from their wounds, police captain Mohamed Hussein said.

The Information Minister said the blast was the largest the city had ever seen.

"It's a sad day. This how merciless and brutal they are, and we have to unite against them," he said, speaking to the state-run radio station.

In a tweet, UN Secretary-General Antonio Guterres said he was "sickened" by the attack, and his spokesman urged all Somalis to unite against extremism and work together to build a "functional" federal state.

The Government has blamed the militant group Al Shabaab for the blast. ( Reuters: Feisal Omar )

AP