"The bomb was buried under garbage. It was targeting a government official who frequently uses that route," Mohamud Hussein Daud, a truck driver, said.
In the town of Baidoa, four people were killed and six were wounded on Thursday evening when a grenade thrown into a crowd watching a foreign film exploded, witnesses said.
"The hand grenade was thrown at a video show in Baidoa late last night," said Mohamed Ali, a resident.
"The [conference] postponement doesn't in any way represent a problem. It shows they want to hold it in a way that will be truly effective"
Michael Ranneberger, US ambassador to Kenya
He did not know the identity of the attackers.
"Four people died and six were wounded. The video show was full of people watching an Indian movie at the time."
Many fighters opposed to Somalia's interim government are hostile to western and Indian films, which they say promote immorality.
The latest attacks come shortly after the postponement of a national peace conference that had been due to start on Thursday.
The US ambassador to Kenya, who is also responsible for Somalia, handed over $1.25m to help pay for the conference and played down its postponement.
"The postponement doesn't in any way represent a problem. It shows they want to hold it in a way that will be truly effective," Michael Ranneberger said in Nairobi.
The planned peace talks will attempt to overcome deep divisions that have fuelled conflict between Somalia's interim government and opposition fighters.
In April, Ethiopian troops supported Somali government forces to combat operations led by the Union of Islamic Courts [ICU] in Mogadishu.
Attacks on Ethiopian troops, government officials and troops from the African Union [AU] have continued in the capital, blamed on ICU fighters and clan members opposed to the interim government.