MOGADISHU (REUTERS) - Islamist gunmen stormed a popular beachside restaurant in the Somali capital Mogadishu on Thursday, setting off two car bombs and battling government soldiers trying to flush them out.

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.

"We are inside and control the cafe," Sheikh Abdiasis Abu Musab, spokesman for al Shabaab's military operations, told Reuters.

"There are many casualties lying inside and outside the cafe."

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.

"The second car bomb has just exploded and the fighters are still inside," Major Farah Abdulle, a police officer at the scene, told Reuters.

"We cannot know the number of casualties inside."

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.