Somalia's Islamist Shebab militants killed at least 19 people when five gunmen detonated a bomb before storming a popular seaside restaurant in the capital Mogadishu, police said on Friday.

"They killed nearly 20 people, including women and children," Somali police officer Mohamed Abdirahman said, describing it as a "barbaric and brutal attack against innocent civilians".

The Lido beach area in Mogadishu is busy with restaurants, including upmarket establishments popular with business people and diaspora Somalis who have returned home to the city.

After a huge explosion, gunmen burst into the restaurant on Thursday evening as diners were sitting down for their evening meal, at the start of Somalia's weekend.

"The fact that they have chosen this location during a weekend night shows how merciless the Shebab militants are," Abdirahman added.

"They wanted to kill more civilians -- but the security forces rescued most of the people."

The Al-Qaeda-linked Shebab claimed responsibility for the attack, in which four of their gunmen were also killed, and one captured.

"The mujahedeen fighters targeted the Lido Beach," the militants said on the group's Radio Andalus, calling it a "major operation against the enemy of Allah".

Security minister Abdirazak Omar Mohamed told reporters that four of the attackers were shot dead by the security forces.

The minister added that one gunman was captured when security forces entered the building to end the attack.

Those who survived described how the gunmen sprayed those in the restaurant with automatic weapons.

Abdirahman Halane was close by the restaurant when the gunmen attacked, and said some of those killed were hit by an explosion as they tried to flee.

"The gunmen entered the popular Lido Sea Food Restaurant from the back door and started shooting... a few minutes later there was a heavy explosion at the front gate while people were trying to escape," Halane said.

"There was confusion, everybody started panicking, and some of them ran towards the gunmen and they were shot dead, I was lucky to escape."

Another witness described how some of those killed had died in the large blast, while others were shot dead at point blank range.

"There is a big mess here and many people died, I saw the dead bodies of 19 people who were killed during the attack," said Hussein Ali, who saw the bodies being carried out of the damaged building. "Some of the people died in the explosion while fleeing and others were shot dead up close."

The Shebab, who are fighting to overthrow the internationally-backed government in Mogadishu, carry out regular attacks in the capital.

A Somali soldier looks at the Lido beach from the terrace of the Lido seafood restaurant on January 22, 2016 following an overnight attack. Photo: AFP