The car exploded next to an Italian cafe on a key thoroughfare in the centre of the city which leads to the presidential palace, in the latest such attack in the violence-scarred country.
"For the moment we have six dead in the explosion, civilians. The car full of explosives blew up next to an Italian cafe," said police officer Mohamed Abdulahi who was at the scene.
"The blast was very powerful and there were a lot of people there at the time, I saw several people dead and injured," added a witness, Abdukadir Ise.
Shabaab claimed responsibility in a statement on a website it habitually uses. The group added that an officer was among the fatalities -- which could not be immediately confirmed.
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"Shabaab fighters were behind a car bomb attack on members of the security forces, the army and immigration services", who regularly frequent the cafe, the group said.
In February Shabaab threatened to escalate attacks in a "vicious war" against the new government of President Mohamed Abdullahi.
The group attacks government, military and civilian targets in the capital and elsewhere, often deploying suicide bombers, and has overrun several military outposts, massacring soldiers from the 22,000-member African Union Mission in Somalia, known as AMISOM.
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