By Jane Hughes BBC correspondent in Gorak Shep, Nepal

Some 35 runners have taken part in the world's highest ever marathon, setting off from Everest Base Camp to mark the 50th anniversary of the first successful ascent of the mountain.

It is generally regarded as a considerable achievement to walk to the base camp, at an altitude of more than 5,000 metres.

Runners have to cross two glaciers

The air there holds just half the oxygen it does at sea-level, leaving people gasping for breath after attempting the slightest incline.

In sub-zero temperatures the runners were already looking tired as they tackled their first hill.

The Everest Marathon took them 42 kilometres (26 miles) through the Himalayan mountains to the town of Namche Bazar.

For most people that is a good two or three days' walk.

But the organisers expected the first runners to finish in around three-and-a-half hours.

And though the runners ended lower than they began, the route is by no means all downhill - there are several big ascents.

On top of that they had to cross two glaciers and to tackle extremely steep and rocky paths.

Honour

The frontrunners were locals accustomed to the altitude.

But others have travelled to Nepal from around the world.

A Canadian, Steve Foster, said this was his first ever marathon, and in such thin air the race was "definitely very challenging".

The organisers say this is an enterprise in the spirit of the original ascent of Everest and is a fitting way to celebrate its 50th anniversary.

The Nepalese Government is marking the occasion by conferring honorary citizenship on Edmund Hillary, one of the first Everest climbers who reached the top of the 8850-metres peak along with a Sherpa climber, Tenzing Norgay.

A home ministry spokesman said that Mr Hillary would be awarded the citizenship next week, to coincide with the golden jubilee of the first Everest ascent.

He is due to attend the golden jubilee celebration in the capital, Kathmandu, and will be the first foreign national to receive such an honour.