No group claimed responsibility for the blast which follows last week's Al Shabab-claimed attack on a hotel in Mogadishu which killed 15 people.
At least 18 civilians were killed when a roadside bomb went off on Thursday in Somalia's Lafole town, southwest capital Mogadishu, blowing up a packed mini-bus that was passing by, police said.
There was no immediate claim of responsibility.
"All the 18 people on board the mini-bus are dead and burnt. A remotely controlled bomb along the road exploded," said Abidkadir Mohamed, a police officer at the scene.
Nur Ahmed, who was driving along the same road, said the mini-bus was being escorted by a vehicle carrying troops.
A government truck full of soldiers followed by the minibus overtook him at high speed before he heard a loud explosion, Ahmed said.
"The government car which was probably the target escaped undamaged," he said.
The Al Qaeda-affiliated Al Shabab militant group have frequently launched attacks against security forces and civilians around the country in the past.
The blast follows last week's Al Shabab-claimed attack on a hotel in the Somalian capital Mogadishu which had killed 15 people including civilians, guards and militants.