Islamist gunmen have stormed a popular beachside restaurant in the Somali capital Mogadishu, setting off two car bombs and killing at least 19 people, police say.

Al Shabaab, a militant group which is aligned with Al Qaeda, claimed responsibility for the attack on the Beach View Cafe on Mogadishu’s popular Lido beach.

“The fact that they have chosen this location during a weekend night shows how merciless the Shabaab militants are,” he said.

Police said Al Shabaab fighters set off the first car bomb at dusk.

A huge second blast, which witnesses said echoed around Mogadishu city centre, struck about an hour later as government soldiers laid siege to the restaurant.

Police officer Osman Nur said the operation ended at 3:00am (Friday, local time).

Security forces kill four militants, capture one

Vehicles were blown up outside the beachside restaurant. Photo: AAP

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,” Mr 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.”

Al Shabaab wants to topple the Western-backed Government in Mogadishu and impose a strict version of Islamic law across Somalia, a nation racked by conflict since the outbreak of civil war in 1991.