The fighters are suspected to be a mix of Islamic courts men and clan militia fighting for control of the capital which, like the rest of Somalia, has been deprived of effective central government since dictator Muhammad Siad Barre was toppled in 1991.
"The favour of Western countries is vital to Somalia's future"
Angus Hogg, UK
Send us your views
Islamic courts fighters, who controlled much of Somalia, were driven out by Somalia's transitional government forces backed by Ethiopian troops some weeks ago.
A cargo plane dropped off 35 uniformed Ugandan officers in Somalia on Thursday, as part of a proposed 8,000-strong Afrian Union force.
Friday's mortar attacks were a reminder of the formidable task awaiting the AU mission intended to help Somalia's government pacify the anarchic country.