Islamic militia and fighters loyal to a US-backed warlord alliance pounded southern and northern Mogadishu with heavy machine gun, rocket, artillery and mortar fire, sending the death toll soaring and thousands fleeing for safety.
At least 30 people were killed and 72 wounded in four residential districts where the fighting was most intense and hospital officials said that the death toll was probably far higher.
The city's two main hospitals reported that eight patients injured in the fighting had died of their wounds and medical officials said they feared the casualty toll could be significantly higher.
At least eight civilians were killed and 29 wounded in the southern K4 neighbourhood, while at least four were killed and four wounded in the northern Sisi district.
Fierce clashes also erupted in southern Daynile and northern Galgalato neighbourhoods, and there were fears the fighting would spread to other areas of the already bullet-scarred city as the two sides erected new roadblocks.
Eight people, three of them civilians, were killed in Galgalato and 17 wounded, while two civilians were killed in Daynile when a mortar landed on their home.
UN calls for ceasefire
Alarmed, the United Nations renewed calls for an immediate ceasefire and expressed deep concern that children were being recruited to fight with weapons supplied in violation of a UN arms embargo.
"I appeal to all parties to immediately and unconditionally hold their fire and respect the needs of the people," said Francois Fall, UN chief Kofi Annan's special envoy for Somalia.
Elders desperately tried to contact commanders from the factions to secure a ceasefire but neither side appeared willing to relent and desperate residents sought shelter from indiscriminate shelling.
"Mogadishu is a ghost city that should be abandoned, people cannot live here in a civilised manner," said Khadija Mohamud, 50, who was leaving her home in the Medina enclave of south Mogadishu.
"We are fed up with this fleeing, this is the third time in a month I have been forced to leave," said Nur Daud, a carpenter, blaming both sides for using excessive firepower that has claimed lives and property.
Thursday's battles erupted after a tense week-long lull in fighting that began in Sisi on May 7, killing more than 140 people over the eight days before the two sides began observing a tenuous ceasefire.
Those clashes, the third major battle between the Islamists and the warlords since February, brought the death toll to more than 240 in the deadliest violence Mogadishu has seen since Somalia collapsed into anarchy in 1991.
The fighting pits the Islamists against the Alliance for the Restoration of Peace and Counter-Terrorism (ARPCT), which was set up in February with US backing to curb the growing influence of Islamic courts and track down extremists, including Al-Qaeda members, they are allegedly harbouring.
The courts, which have declared a holy war against the alliance that they say is financed by the "enemy of Islam," deny the accusations.
Somalia's largely powerless transitional government, based in Baidoa about 250 kilometres northwest of Mogadishu, has blamed both the alliance and the United States for the fighting.