Islamic extremists detonated four bombs outside a hotel in the capital of Mogadishu on Friday afternoon, killing at least 20 people and injuring 17, said police.

After the three explosions in front of the hotel, a fourth blast hit as medics attempted to rescue the injured.

The suicide bombs detonated near the perimeter wall of the Sahafi Hotel, which is across the street from the Somali Police Force's Criminal Investigations Department, said Capt. Mohamed Hussein.

Some of the victims were burned beyond recognition when one car bomb exploded next to a minibus, he said.

Somali security forces shot dead four gunmen who tried to storm through a hole blown into the hotel's wall, but did not succeed in entering, he said.

"Although they failed to access the hotel, the blasts outside the hotel killed many people," said Hussein.

"The street was crowded with people and cars, bodies were everywhere," said Hussein Nur, a shopkeeper who suffered light shrapnel injuries on his right hand. "Gunfire killed several people, too."

Somalia's Islamic extremist rebels, al-Shabaab, claimed responsibility for the bombs, according to the group's Adalus radio station.

Among the dead was the manager of the Sahafi Hotel, whose father was its owner before he was killed in an al-Shabaab attack in 2015, said Hussein.