* Libya struggling to curb clout of regional militias
* Clashes still common two years after Gaddafi's fall
* Oil exports blocked by widespread armed disorder
At least 13 people were killed and more than 130 wounded in fighting between militiamen and armed residents of the Libyan capital Tripoli on Friday, the state news agency Lana said - the third outbreak of street fighting in 10 days.
Prime Minister Ali Zeidan demanded that all armed militias leave Tripoli, "without exception".
But the clashes underscored the weak central government's inability to contain the regional militias that helped to overthrow Muammar Gaddafi two years ago but kept their guns. Armed disorder has blocked most oil exports for months.
Friday's bloodshed, the worst in Tripoli for several months, began when militiamen from the city of Misrata fired into the air and then into hundreds of protesters demanding their eviction from the capital after they had fought other groups for control of certain neighbourhoods.
A Reuters reporter saw an anti-aircraft cannon firing from the "Gharghur brigades'" gated compound into the crowd as protesters chanted: "We don't want armed militias!"
Demonstrators fled but then returned, heavily armed, to attack the compound, where the militiamen remained holed up past nightfall.
Dozens of soldiers in trucks tried to separate the two sides, and sealed off roads to prevent more people joining the clashes.
Heavy smoke could be seen rising from the scene in the Gharghur district, where many of Gaddafi's closest collaborators used to live until the uprising.
Air force planes circled overhead. "We want to make sure the militia don't bring in any reinforcements," said army spokesman Ali al-Sheikhi.
Tripoli has been spared the almost daily bombings and killings that plague Libya's second city, Benghazi, in the east.
But when clashes between rival militias do break out, the nascent armed forces are no match for them.
Strikes and armed protests around the country by militia and tribal gunmen demanding payments or more autonomy rights have also shut much of the OPEC member's oil output, depriving the government of its main source of income.
The authorities have tried to defuse the threat of the militias by placing them on the government payroll and assigning them to provide security.
But the gunmen often remain loyal principally to their own commanders and fight for control of local areas, especially their weapons or drug smuggling, or to settle personal feuds.
Zeidan was himself briefly abducted in October by a militia group on the government payroll. He has called for more foreign training for his military.
Friday's violence began as a peaceful rally by some 500 people demanding the departure of the Misrata gunmen, who had fought twice last week with a rival group that had detained one of their members for driving a car without number plates.