By Katrina Manson

BO, Sierra Leone, Aug 30 (Reuters) - Mustapha Sesay and Femi Rashid, former foes in Sierra Leone's civil war, spar with good humoured jibes as they work together in a motorcycle taxi association that brings together ex-combatants.

"I shoot you like a chicken," laughs ex-rebel child soldier Sesay, 22. "You don't know how to fight," retorts Rashid, 40, once a traditional Kamajor hunter who battled the rebels.

This spirit of reconciliation allowed the former British colony in West Africa to successfully hold its first elections since United Nations peacekeepers left two years ago following the 1991-2002 war, one of Africa's most brutal conflicts.

But in the Aug. 11 first round vote for the presidency, no candidate gained the 55 percent required to win.

As Sierra Leone prepares for a decisive run-off next week, recent clashes between rival supporters have raised fears that tensions could reopen old wounds from the war, which saw drugged child soldiers gun down unarmed civilians and hack off limbs.

President Ahmad Tejan Kabbah warned this week he could declare a state of emergency if electoral violence worsened.

Police imposed a brief curfew on Monday in the eastern border region with Liberia, a centre of the illegal diamond mining trade which fuelled the civil conflict.

U.N. Secretary-General Ban Ki-moon was "concerned about the tensions and violence that have been increasing in Sierra Leone since last week", his spokesperson said on Wednesday.

"He calls on all parties and their leaders to do everything necessary to prevent the situation from escalating," she said.

The contenders for the Sept. 8 run-off, main opposition candidate Ernest Bai Koroma of the All People's Congress (APC) and Vice President Soloman Berewa of the ruling Sierra Leone People's Party (SLPP), appealed for calm.

Addressing supporters later, Koroma criticised the president's warning as an electoral ploy aimed at damaging the opposition. "He's got no business declaring a state of emergency. They are so desperate," he said.

VOLATILE EX-COMBATANTS

Koroma won 44 percent of the vote in the first round, while Berewa polled 38 percent.

Koroma's chances of victory in the run-off have been boosted by public backing from the third-placed first round candidate, Charles Margai, who won 14 percent.

International observers hope Sierra Leone can preserve the unity and reconciliation exemplified by former enemies like Sesay and Rashid, who now work together running the Bo Bike Riders' Association.

It represents nearly 5,000 motorcycle taxi drivers throughout south Sierra Leone, mostly ex-combatants.

"Once they were fighting, and now they are working together ... It is wonderful," said David Ngombu from international aid organisation Conciliation Resources.

More than 70,000 fighters were disarmed at the end of the war, each given the equivalent of $100 to hand in their guns. Many drifted into driving motorcycle taxis after donor-funded skills training schemes closed down or ran out of money.

But making a living can be tough for the ex-combatants and the association tries to keep them employed. "An idle man is a devil's workshop. The bike riders can be very volatile," said the group's legal advisor Solomon Rogers.

Sesay, who was abducted to fight by rebels when when he was 12, says he will not go back to war.

"We only fight with this now," he said, pointing to his thumb, which illiterate voters use to mark their ballot choice.

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