MOGADISHU, Somalia — Islamic insurgents fired mortar rounds at Somalia's presidential palace and clashed with government forces Wednesday, leaving at least five civilians dead a day after Ethiopian troops handed over security duties.

The violence underscored fears that Somalia could collapse into further chaos following the Ethiopians' departure, with extremists moving to seize power from the country's weak U.N.-backed government.

Government soldiers retaliated after Wednesday's attack on the palace, and some of their mortar rounds hit the capital's largest market of Bakara, said Farah Mumin, a salesman at the market who said he saw three civilians killed and nine others wounded.

"The scene was very horrific and everyone ran from the market, some of them leaving their shops open," said Fadumo Sahal, another witness.

Elsewhere, two male teenagers were killed when a mortar struck them as they ran to seek cover in a building, said Dahir Absuge, a resident who saw what happened from his house.

The violence comes a day after neighboring Ethiopia handed over security duties following a two-year deployment in Somalia. The Ethiopian army, one of Africa's largest, was viewed by many Somalis as abusive and heavy-handed.

But few expect the Somali government can ensure security even with the help of the Islamist faction with which it has agreed to share power. The government controls only pockets of the capital, Mogadishu, and Baidoa, where parliament sits — and has tried to rule without a president for weeks.

Meanwhile, Somalia faces the Islamic insurgency and rampant piracy off its coast. In the past year, thousands of civilians have been killed in fighting, particularly in the capital, and hundreds of thousands have fled the violence.

It was unclear when all the thousands of Ethiopians will have departed. They were pulling out in stages and gave no exact dates for security reasons. Residents said Ethiopian troops had vacated two bases on Wednesday.

Separately, Islamic insurgents attacked other Ethiopian troops withdrawing from a key road junction in southern Mogadishu. Insurgents and Ethiopians rarely comment on their casualties.

Somalia has not had a functioning government since 1991, when rival warlords overthrew dictator Mohamed Siad Barre and then turned on each other.

Its weak U.N.-backed government had called in the Ethiopian troops in December 2006 to oust an umbrella Islamic group — which included the al-Shabab extremists at the center of the current fighting — that had controlled southern Somalia and the capital for six months.