* Clashes have erupted in south, west Libya
* Long-standing rivalries, weapons fueling violence
* Government struggling to impose authority
By Marie-Louise Gumuchian
SABHA, Libya, April 8 (Reuters) - When 26-year-old Yussef heard about clashes in the Libyan oasis city of Sabha, he wasted no time in picking up a rifle and heading across the desert to support his fellow Tibu fighters.
Yussef said he felt compelled to make the 150 km drive from the southern town of Murzuq to stand up for his Tibu people, a black African ethnic group, when it fought a local Arab militia for six days last month.
"There is no strong government to protect us, our families," he said, pointing to bullet-scarred buildings in a mainly Tibu neighbourhood of Sabha. "I had to come to help my people defend themselves," said Yussef, who declined to give his full name.
While the opposing sides agreed to end the fighting after about 150 people died, residents fear it will reignite even though the army was sent to restore calm in the desert city.
Long-standing rivalries, divided communities and plentiful weapons are convulsing Libya as the interim government struggles to impose its authority and secure peace among the country's ethnic groups.
Violence in the Saharan south and in western Libya have shown how volatile the country remains six months after the overthrow of Muammar Gaddafi, who had long played off one tribe or clan against the other to weaken their power.
Using threats of violence, awarding economic privileges and exploiting family loyalties to divide and rule, Gaddafi ensured that tensions between rival groups persisted.
In the south, where tribal ties are far more powerful than on the Mediterranean coast, porous borders with neighbouring countries, discontent and the availability of arms make the region one of the biggest potential problems for the government.
Gaddafi was always quick to end any ethnic clashes in the desert region and in 2009 he put down a tribal rebellion with helicopter gunships. But Libya's new rulers are weak and often out-gunned by the militias they are trying to control.
Like most of the confrontations that erupt in Libya, the Sabha clashes began with an incident which quickly escalated in the absence of any proper security force.
Oasis farmers by tradition, the Tibu supported the rebel side in last year's uprising against Gaddafi. Their opponents are ethnic Arabs who see the Tibu, a group that also lives across the border in Chad and Niger, as outsiders.
Sabha still bears the scars of the fighting and residents fear it could resume at any time.
In the mainly Tibu district of Tayuri, bullet cases lie on the sandy ground. Large holes gape in ceilings of small single-storey homes, some of them shacks, standing empty after families fled to nearby villages. Walls are riddled with bullet-holes.
At a small neighbourhood clinic, plastic gloves and bloodied bandages remain scattered among the debris. "There is peace now but people are afraid," said Khadija Issa, a nurse at the clinic. "The fighting could start up again."
STRUGGLE FOR POWER
In an attempt to patch up the dispute, Prime Minister Abdurrahim El-Keib flew to Sabha to show the opposing sides that the new Libya had a place for all tribes and ethnic groups. However, hecklers accused him several times being slow to intervene to stop the clashes.
Restoring order in the south is important to the stability of the wider region. In the chaos since Gaddafi's fall, the south has become a smuggling route for weapons which are reaching al Qaeda militants deeper in the Sahara and Tuaregs who are staging a separatist rebellion in northern Mali.
It is also used for trafficking legal and contraband goods such as alcohol, cigarettes and drugs, and by African immigrants heading north in the hope of reaching Europe.
In February, a long-standing rivalry between the Tibu and the Arab Zwai tribes erupted into violence in Al Kufra over control of territory. In Sabha, the fighting began after a man was killed in a carjacking, residents said.
"This fighting is about a struggle for power," a government source said. "We need a solution for the south."
Libya had a unique system of government under Gaddafi. His Green Book of political ramblings served as a de facto constitution, while legal disputes were often settled in tribal talks rather than in court.
With Gaddafi gone, restoring order and enforcing a new political system is particularly daunting in dusty outposts such as Sabha or Al Kufra where central authority appears distant.
"If we had the power we could manage to solve the problem from the beginning," said Ayub Azarouq, who heads Sabha's local council. "Power means we should have a police, an army. As long as there is no official army, we can still be weak. We should have an official army to solve the problems."
Without a genuine army, Libya's National Transitional Council has struggled to persuade the many militias who fought Gaddafi across the country to join the armed forces and police, limiting its ability to intervene quickly in troublespots.
Initially the NTC relied on former rebel fighters from other cities to restore calm in Sabha. The government in Tripoli has since sent soldiers and named a military governor for the south. But while residents welcomed their arrival, tensions remain.
"It is the Tibu who are responsible for the violence here," one Sabha resident said, declining to be named for fear of being recognised. "They have a different mentality from us."
With its hands full keeping the peace in Sabha, the government faced a new crisis this week when fighting erupted around Zuwara, about 120 km west of Tripoli.
This violence has its roots in the rebellion. Zuwara's population of largely ethnic Berbers opposed Gaddafi last year while their Arab neighbours in settlements to the south remained loyal to the Libyan leader.
Government officials have denied the recent fighting has ethnic or tribal roots but say insecurity could delay Libya's first free elections to a national assembly in June.
Analysts say the fighting will not tear the country apart, but it underlines the insecurity. "There is very little the government is currently able to do to prevent further clashes from occurring, either in the same locales or in new ones," Geoff Porter of North Africa Counsulting said.
"That said, the fighting does not pose an existential threat to the viability of the country. The fighting has not been motivated by explicit grievances with the state, it is not driven by secessionist tendencies, and it does not appear to be underpinned by political ideology.
"Were the fighting to evince any of these characteristics then it would be much more troubling and pose a greater risk to Libya's prospects as a coherent sovereign nation."
Disarming the thousands of rebel fighters across the country will be vital if the violence is to be controlled, but the huge amount of weapons in circulation means results will take time.
Sabha residents want faster action. Dozens of women took to the city's streets this week calling for a stronger army. The prime minister, an academic who lived abroad before his appointment in November, was not tough enough, they said.
"We respect El-Keib as a person but he is someone with a Western mentality. The Libyan society needs someone firm," 50-year-old teacher Khadija Taher said. "The government should adopt a better policy or else we will have another revolution." (editing by David Stamp)