The Omo valley in Ethiopia could be the last wilderness in Africa.
"You like Utopia?" The first and most enduring impression of Ethiopia is an endless series of friendly faces cheerfully mispronouncing their country's name. Some of these faces belong to handsomely attired Orthodox priests, some to white-robed pilgrims, some to shepherd boys, some to tall, beautiful women with beads in their hair. But in the Omo valley, a remote area in the far south west near the Kenyan and Sudanese borders, the people have only recently become aware that such a thing as Ethiopia exists, and they ask a different question.

"You have razor blade?"

I eyed the ridged network of garish scars decorating their bodies and flapped a hand rather urgently to our guide, Alex, who ambled over with a grin. As usual he was prepared for anything, and handed me a bag bulging with strings of brightly coloured beads: much better for the skin, and the Karo people loved them. We were instant hits. Faces lit with humour, the Karo draped the beads around their wrists and necks and started to prod and poke us gently.

The bright-eyed and endlessly curious Karo are one of the most threatened tribes of the lower Omo. They number fewer than 1,000, and a single epidemic could obliterate them; but more worrying are other tribes in the area, who are not always friends. The Mursi are permanently at war with the Hamar, the Hamar with their cousins the Karo; and the Bumi fight both the Karo and the Surma whenever they can. The women scarify their chests to beautify themselves, but the men do so to indicate their having killed an enemy or a dangerous animal.

Few areas remain in Africa that can be described as true wilderness, but the valley of the wide, smooth-flowing Omo River is a good candidate - a lost world, rendered almost uninhabitable by the tsetse fly. Here spectacular landscapes are unblemished by man, and huge herds of eland and buffalo roam across the unspoiled savanna grassland, along with giraffe, elephant, zebra, lion and leopard. Unused to tourists, the animals are wild and shy, but despite having been warned that game was far less visible than in other East African countries, we were not disappointed.

Alex, a professional hunter, has an infectious enthusiasm for the country as well as an impressive knowledge of its wildlife, and cheerfully woke us for each morning's safari at 5am. Spotting wildlife in the grey light of dawn was extremely difficult. Alex patiently pointed to a clump of dry, spindly bushes in the distance. Yawning, I picked up my binoculars to see the two enormous brown eyes of a gerenuk - a slim antelope - staring straight at me. Next he waved at a dark smudge on the horizon, which turned into a massive herd of tiang - related to wildebeest - browsing peacefully.

Gradually, as I became more aware of what to look for, curved sticks poking into the skyline materialised into oryx, rocks into baboon, and bushes into kudu, waterbuck, oribi and hartebeest. The bird life was magnificent: we saw secretary birds, bustards and great flocks of carmine bee-eaters as well as an abundance of vultures and birds of prey. By mid-morning the game would have vanished, to find shelter from the broiling heat, and we would return to camp.

The Omo camp is currently the only permanent one in the area, and is set on the banks of the river, pitched in the shade of tall trees occasionally inhabited by a colony of colobus monkeys. Billed as a luxury camp, it has all the required trappings: twin beds, flush lavatories, pedestal basins and hot showers, housed beneath a dozen neatly thatched roofs. This is usually a hunting camp, but the owner is happy to accommodate tourists on photographic and walking trips. It is possible to hire a vehicle and driver and travel independently, but the roads are rough and an experienced mechanic and a supply of spare parts are a must. We had three flat tyres and needed to change our fuel filter twice during our two-day drive back to Addis Ababa.

Safaris in the early evening are perhaps the most idyllic. The air is warm and soft as velvet, the sky a hazy lavender. Standing high on the back of our jeep, we bounced across the baked brown grasses, negotiating herds of gazelle and tiang.

As night drew in, Deganu, our tracker, scanned around the vehicle with a spotlight. Suddenly another world took shape: the day shift of antelope was transposed to the night shift of predators. Our first excitement was spying a genet cat, a miniature leopard with a tail as thick as a man's wrist; then a civet cat darted behind some acacia trees. Fresh hyena tracks padded down the dusty track and African barn owls swooped overhead. More sinister were the dozens of red eyes that glimmered malevolently along the softly silted banks of the Omo, and after counting 18 crocodiles opposite our camp I wondered about the group of Italian tourists (the only other tourists we saw) we'd spotted earlier swimming in the river.

The most vivid memory is the sighting of my first leopard. At 4am, we were huddled shivering in the jeep, the air chill, when Deganu hissed excitedly: "Leopard, leopard, leopard." It was pitch dark and he swung the lamp to the right as Alex stamped on the brakes. Suddenly a liquid silk form of black and gold froze in its tracks, just yards away. The leopard's eyes stared at us, stunned, captured by Deganu's lamp for perhaps 10 seconds, and then the powerful muscles flexed and he sprang into the blackness.

We'd risen earlier than usual in order to visit the Mursi tribe, who are renowned for their practice of inserting large, circular clay plates behind the lower lips of their women. These are purely symbolic, and the size of the lip plate determines the size of the bride price. A large lip plate, for example, will bring in 50 head of cattle. And 49 cattle can buy seven Kalashnikov rifles, the most significant currency of all in this part of the world.

It was a six-hour bone-rattling drive to the nearest Mursi village, punctuated briefly by stops to view buffalo, warthog and lesser kudu. There were ample antelope but giraffe were scarce; not only are they easier to kill than other game, but the Mursi relish their meat. On the edge of the Mago National Park we spotted the bleached remains of a dead elephant, picked so clean that it appeared to have been there for a decade, not just a month. The Mursi, Alex told us, were partial to elephant meat, too.

Sweat-drenched and covered in insect bites, we finally arrived at a clutch of thatched huts shimmering beneath the searing midday sun. Two tall warriors stood on the horizon with their spears, but otherwise all was still, silent. Then, the instant our jeep entered the village compound, the silence gave way to a deafening uproar.

Swarms of people erupted out of the huts and raced for us, shouting at top volume, waving their arms furiously. The next second we were surrounded by an agitated, excited crowd who grabbed at us, yanking our T-shirts and pinching our skin, their eyes feverish. "Photo, photo, photo!" they shrieked, fighting each other to get to us, thrusting warthog tusks into our pockets and clay plates into our hands. "Ten birr. Ten birr." They snapped their fingers in our faces, refusing to take their wares back unless we paid for them.

The deal when you visit the Mursi is that if you take a photograph of them, you either pay them or give them a present. Of course, every one of the 60 villagers wanted a gift for themselves, one for their mother and one for each of their children, and the whole event rapidly turned into a bun fight. At one point a six-foot tall woman was tugging my right hand, while another woman was pulling my left in the opposite direction, both yelling "photo!" at the tops of their voices. A small boy, about five years old, clung limpet-like to my left calf.

When Alex started the jeep's engine I was beside him before you could say "10 birr". Because no matter how statuesque the women, how endearing the children, one hour of the Mursi was quite enough.

As we roared off we passed a land cruiser full of Americans coming the other way. I was surprised to learn that, on average, three vehicles visit the Mursi each day, and that most tourists undertake the exhausting trip to the Omo valley purely to see the Mursi. I hoped they weren't going to be disappointed.

Ethiopian Airlines (0171-491 9119) flies five times a week from Heathrow to Addis Ababa. Through Tradewings (0171-631 1840) a return ticket costs pounds 561 including tax. Bridge The World (0171-911 0900) sells flights via Cairo on Egyptair for pounds 454. British visitors can get a visa from the Ethiopian Embassy at 17 Princes Gate, London SW7 1PZ (0171-589 7212). Trips to the Omo valley are run by Ethiopian Rift Valley Safaris, PO Box 3658 Addis Ababa (00 251 1 55 1127).