Roadside bombs are often used by insurgents fighting the government
A roadside bomb has killed at least 20 people in the Somali capital, Mogadishu, witnesses have said.
Most of the dead in the explosion were women who had gathered to clean the streets, they said.
Reports say the bomb was hidden under a pile of rubbish on the main road to Mogadishu airport.
There was no immediate claim of responsibility, but tensions are high as Somalia's government and Ethiopian troops battle Islamist insurgents.
Roadside bombs are frequently used by insurgents fighting the transitional government and its Ethiopian allies.
"They were cleaning the street when this huge explosion rocked the entire neighbourhood," eyewitness Hassan Abdi Mohamed told the AFP news agency.
"I counted 15 bodies, most of them are women who were torn to pieces," he added.
Forty-seven wounded people - mostly women and children - were admitted to Mogadishu's Medina hospital, officials said.
The UN says 1.5 million people have fled after recent fighting.
Somalia's opposition groups, which include Islamists ousted from power in 2006, are opposed to Ethiopian troops remaining in Somalia.
According to one estimate, more than 8,000 civilians have been killed and one million forced from their homes since the start of last year by fighting between the interim government and the insurgents.
African Union troops have not been able to quell the violence, which has triggered what aid workers say may be the worst humanitarian crisis in Africa.
Somalia has been devastated by conflict since 1991, when former President Mohamed Siad Barre was ousted.