More than 10,000 people fled the Somali capital in the last week alone and the exodus sparked by fighting continues, the UN said.
The UN High Commissioner for Refugees (UNHCR) said on Monday that even those who had returned to Mogadishu recently were heading out again, fearing for their safety.
"The prospect of still another round of heavy fighting has set many civilians on the road once more," the agency said in its statement.
The UN yesterday appealed for US$48 million to help those fleeing renewed violence and said the overall number of people uprooted in the country was estimated at 500,000.
The funding appeal aims to provide aid to internally displaced Somalis and Somali refugees crossing into Kenya, Ethiopia, Yemen and Djibouti through the end of next year, a UNHCR statement said.
Mogadishu has seen little peace since government troops backed by Ethiopian forces drove an Islamic movement out of the city in December. Roadside bombs, attacks on government installations, assassination attempts and gunbattles have become common, and civilians are often caught in the crossfire.
Fighting in March and April forced about a fifth of Mogadishu's 2 million residents to flee for safety and only about 125,000 have returned, UNHCR said.
"It is really a horrible place to be," said resident Said Dahir Haji Igal, holding two of his children by the hand.
"I have no option but to flee for the safety of my children," he said.
The majority of those fleeing the capital were from districts near the hall hosting a national reconciliation conference aimed at helping the country heal the wounds of 16 years of conflict. The meeting was delayed several times due to violence and infighting, and has been the target of regular insurgent attacks since it opened on July 18.
Raho Yusuf, 19, said she had to leave her house near the conference site but had nowhere to go.
"I have no money for transportation. I therefore have resorted to live on the streets asking for a help from other Somalis," said Yusuf, who has a two-year-old daughter.
Mohamed Ali Heyle, 60, said he was "on the verge of starvation."
"Our days are numbered because we don't have enough food and water," he said.
The international aid group Doctors without Borders said on Monday that most of those displaced took refuge in Afgoye and Hawa Abdi, some 30km outside Mogadishu.
"One of the major problems we have found is lack of food," said Fernandez Gustavo, head of the group's mission in Somalia.
He said a recent survey showed 93 percent of people in Afgoye have no source of income.
The Somali insurgents are linked to the Council of Islamic Courts, which ruled much of southern Somalia for six months last year.
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