* Underscores fragile security after militants expelled
* One bomber walked into popular hangout, car bomb outside
* Islamists al Shabaab have threatened attacks (Adds witness, details, background)
By Feisal Omar
MOGADISHU, March 1 (Reuters) - Suicide bombers killed a civilian at a beach front restaurant in the Somali capital on Friday, officials said, underscoring how fragile security remains 18 months after African troops drove Islamist militants out of the city.
Ali Abdi, a waiter at Mogadishu's Indian Ocean restaurant, said one bomber wearing a vest laden with explosives walked inside and blew himself up moments before a car bomb exploded a few metres away.
"I see blood and human flesh all over the walls and floor. Chairs and tables litter the area," Abdi told Reuters.
The Friday crowds at the Lido beach, where a row of blown-out colonial villas are a stark reminder of two decades of civil strife, have come to symbolise Mogadishu's rebound following the expulsion of the al Qaeda linked militants.
But the city remains plagued by bomb attacks and assassinations and the central government depends heavily on an 17,600-strong African Union peacekeeping force for its survival.
A Somali soldier at the blast site said one civilian had been killed, as well as the bombers.
A witness also reported hearing two blasts in the area, though there was confusion over which bomber detonated his explosives first. A senior police officer said the car bomber rammed the restaurant before the second bomb blast.
There was no immediate claim of responsibility but the al Shabaab rebel group has vowed to carry out guerrilla-style attacks against the new government, which is supported by Western powers and regional states.
The militants controlled much of Mogadishu between 2009 and 2011, during which time the capital's bombed-out streets were the frontline in a bloody battle between the rebels and African peacekeepers.
Before that, another Islamist group controlled the city after seizing it from warlords and militia fighters.
Civil war after the fall of dictator Mohamed Siad Barre in 1991 left Somalia without effective central government and awash with weapons. The turmoil opened the doors for piracy to flourish in the Gulf of Aden and deeper into the Indian Ocean. (Reporting by Abdi Sheikh and Feisal Omar; Writing by Richard Lough; Editing by Drazen Jorgic and Andrew Heavens)