The shelling in the heavily populated market started after al-Shabab insurgent group fired mortars at the presidential palace where the Ethiopian and government troops are based, Shabelle Media Network in Mogadishu reported.
"A mortar shell landed in my restaurant and six people died instantly," restaurant owner Ali Hassan said of the carnage he witnessed. "Nine civilians were seriously injured."
Earlier in the day, Ethiopian soldiers supported by two tanks arrested dozens of Somali teenagers in Mogadishu. It was not known why they were detained.
The shelling followed Monday's fighting in central Somalia and around Mogadishu in which at least 40 Somalis were killed and 50 others wounded in clashes between moderate and hard-line groups of Islamic fighters, observers said.
The violence follows President Abdullahi Yusuf's decision this week to quit his post.
Observers said the violence in central Somalia is coming between a well-known Islamic militant group with alleged ties to al-Qaida called al-Shabab and a previously unknown group of more moderate Islamists called Ahlu Sunna Waljamaca. The latter group has declared a holy war on al-Shabab, blaming it for violence in the country, the BBC reported.
Ahlu Sunna Waljamaca seized two towns in central Somalia during the weekend, triggering bloody clashes.
The weak, pro-Western central government controls only certain parts of Mogadishu and the town of Baidoa. Various Islamist and nationalist groups control the bulk of Somalia, the BBC said.