At least 20 people have been killed and 50 others injured in clashes between government troops and opposition fighters in the Somali capital Mogadishu.

Witnesses said that fighting broke out on Tuesday after an armed group attacked police and African Union peacekeepers in the capital's southern Taleh district.

The fighting came a day after Sharif Ahmed, Somalia's newly elected president, returned to the country from Djibouti.

Witnesses said that heavy machine guns and artillery were used in Tuesday's fighting and that the fighters shelled the presidential palace.

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Several residents said that two civilians had been hit by stray bullets near the scene of clashes and three others were killed when a mortar shell struck a house.

A medical official said that more than 20 wounded people had been transferred to the city's Medina hospital.

The clashes follow Sunday's suicide bomb attack on an AU base in the Somali capital, which left 11 Burundi soldiers dead.

The bombing, for which the al-Shabab armed group claimed responsibility, was the deadliest so far against the African Union Mission in Somalia.