Somali President Mohamed Abdullahi Farmajo. (AP Photo/Farah Abdi Warsameh)

Tens of thousands of people took to the streets in Somalia's capital, Mogadishu, on Wednesday to show their frustration with repeated attacks by Islamist terrorist group al-Shabaab, after a weekend truck bombing killed almost 300 people.

Protesters wore red bands around their arms and heads in support of the families of the victims of the most recent attack. Some carried placards addressed at al-Shabaab that read, "Stop killing innocent people."

"We are telling [al-Shabaab] that we are ready to take up arms and fight them from now on," a 16-year-old protester told dpa in Mogadishu.

The march took the people through the city and to a rally at its Konis satdium, where President Mohamed Abdullahi Mohamed Farmajo vowed the government would soon take revenge on those behind the attack.

Speaking before thousands of protesters, the president called on Somalis to united to kick al-Shabaab out of the country.

"This massacre was far from humanity and shows the ruthless of al-Shabaab. ... We have to unite to defend our people."

He promised the creation of a new task force to take on al-Shabaab.

"We must get ready to go at front lines and I will be the first person to enlist for the new task force" the president said as he laid out plans.

Demonstrations against the extremists also took place in Somalia's volatile Puntland, Jubaland and Galmudug regions.

At least 281 people were killed and 300 others injured when a suicide bomber detonated a truck filled with explosives at a busy intersection in Mogadishu on Saturday. It was the deadliest single attack in the volatile East African nation's history.

Before detonating the truck he was driving, the suicide bomber had raced along a Mogadishu street at high speed, rolling over motorcycles and cars and shunting vehicles stuck in traffic.

Farmajo called the attack a national tragedy.

Police said Wednesday they had made several arrests in relation with the bombing, but did not provide further details.

No group has claimed responsibility for the attack but many suspect al-Shabaab, a militant group affiliated with the al-Qaeda terrorist network, which is seeking an Islamist state in Somalia.

A 22,000-strong African Union force supports the Somali military in its fight against the terrorists.