Militias clash in the centre of Libyan capital Tripoli

Clashes have erupted in the centre of the Libyan capital, Tripoli. Residents report hearing sustained gunfire and seeing casualties, but the scale of the violence is not clear.
It is also not clear who is involved in the fighting or why it broke out, but Tripoli is still dominated by militias of former rebels from outside the city.
The fighting broke out at a building used as intelligence headquarters by the former regime of Muammar Gaddafi, who was killed in October 2011.
AFP news agency reported that militiamen had blocked ambulances from reaching the area of the fighting.