MOGADISHU, Somalia — At least 17 civilians were killed and 56 injured in the Somali capital of Mogadishu on Monday when a soldier fired an anti-aircraft gun into a crowd, witnesses and officials said.

Soldiers had killed a plainclothes policeman, triggering a tense standoff as his heavily armed colleagues came to confront the troops. A soldier then triggered the anti-aircraft gun mounted atop a truck, although it was not clear if it was accidental or intentional.

Witness Soleman Abdirahshid told the German news agency dpa that dozens of people were cut to pieces as high-caliber bullets ripped through the crowd.

“”Everyone was terrified because all you could see were dead bodies littered around,”” he said.

“”Five people died while receiving treatment, while 12 others were dead- n-arrival,”” Mohamed Yusuf Hassan, director of Medina Hospital, told dpa. “”Fifty-six people, most of them seriously injured, were admitted.””

Abdihakin Mohamed Fiqi, Somalia’s Defense Minister, visited the wounded and promised an investigation.

“”We are very sorry for what happened, but we should bring to justice those responsible for the incident,”” he said.

Somalia’s ineffectual government is struggling to contain an Islamist insurgency that has claimed tens of thousands of lives over the last few years.

The mandate of the Transitional Federal Government, which only controls around half of Mogadishu with the support of African Union peacekeepers, runs out later this year with nothing to show for its efforts.

The regional body the Intergovernmental Authority on Development, meeting on the sidelines of an African Union summit in Addis Ababa, said it had agreed the government’s mandate should be extended, although the AU criticized the TFG for performing badly.

“”This nonperformance, if you may, has impacted negatively on perceptions of its legitimacy and credibility,”” African Union Commission Chairman Jean Ping told the IGAD meeting.

Somalia has been engulfed in chaos since the 1991 ouster of dictator Mohamed Siad Barre.