* Misrata has thousands of tonnes of guns, ammunition
* Weapons a challenge to the weak central government
* No timetable for handing over arms to national army
MISRATA, Libya, Dec 7 (Reuters) - Militias outside the control of Libya's central government are holding vast stores of tanks, rockets and small arms in the city of Misrata, an arsenal that will test the ability of the country's new rulers to assert their authority.
A Reuters team gained rare access to militia warehouses in Misrata and counted thousands of boxes of arms and ammunition, most of it seized from forces loyal to ousted leader Muammar Gaddafi and hauled back to the city in trucks.
The militias, which were formed to fight Gaddafi's rule and profess loyalty to the interim leaders of the National Transitional Council (NTC), say they will hand over the weapons once a new national army is created.
But there is no timetable for that and in the meantime the weapons give Misrata more military might than the fragile government in Tripoli, an advantage the Misrata militias are likely to try to convert into political power.
"The government does not have a monopoly on force in the country," said Geoff Porter, a north Africa expert who has testified on Libya in the U.S. Congress "Without it, the state's ability to function is jeopardised."
"All of the militias are amply armed and the government has no recourse but to urge and cajole them to give up their weapons," he said.
Over two days, Reuters reporters visited four weapons stores operated by three of the city's militia brigades.
This offered a cross-section of the weapons in the city but represented only a fraction of the total: Misrata has six brigades, with between them more than 200 units. Most brigades have several weapons stores in different locations.
The weapons that could be seen included, according to a Reuters count, 38 tanks, nine self-propelled guns, 16 field guns, 536 Russian-made Grad rockets and 13 truck-mounted Grad launchers, 2480 mortar rounds and 202 artillery shells.