A roadside bomb has killed at least 15 people in the Somali capital, Mogadishu, witnesses say.The explosion hit a group of women gathered to clean the street, witnesses said. They told of bloody scenes with several dozen people injured.One witness told the AFP agency most of the dead were female street cleaners.There was no immediate claim of responsibility, but tensions are high in Mogadishu as Ethiopian troops battle Islamist insurgents."They were cleaning the street when this huge explosion rocked the entire neighbourhood," eyewitness Hasan Abdi Mohamed told the AFP news agency."I counted 15 bodies, most of them are women who were torn to pieces," he added.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.