* 19-year-old claims upset win

* First world championship gold for Eritrea (Adds detail, quotes)

By Nick Mulvenney

BEIJING, Aug 22 (Reuters) - Eritrean teenager Ghirmay Ghebreselassie became the youngest ever marathon world champion when he stunned a strong field of fellow East Africans to win the first title of the Beijing championships on Saturday.

The 19-year-old eased clear of the field over the final six kilometres and crossed the line in two hours, 12 minutes and 27 seconds to claim a first world championships gold medal for his country.

"I cannot explain what I am feeling, I am so happy," he told reporters.

"I am so proud of this victory. My medal is the first gold medal in Eritrean history. It is a very special thing for our history.

"My parents wanted me to be a great student but I wanted to be a great athlete. Today's victory will be a great surprise to them."

Ethiopian Yemane Tsegay, who led briefly before Ghebreselassie's final surge, finished 40 seconds behind the Eritrean in second place, while Uganda's Solomon Mutai claimed bronze in 2.13.29.

Ruggero Pertile of Italy, who led for much of the middle part of the race, finished fourth, while Uganda's Olympic champion Stephen Kiprotich gave up the world title he won two years ago from sixth place.

After an early morning start to mitigate the effects of the Beijing summer heat, Tsegay and Bahraini Shumi Dechasa set the pace in the early stages as the runners passed the sights of central Beijing.

Pertile and his compatriot Daniele Meucci had taken over at the front by the 20 kilometre mark and they maintained their lead until Lesotho's Tsepo Mathibelle took off at a sprint 10 kilometres later.

It was a pace that would prove impossible to maintain in the punishing humidity, however, and Ghebreselassie reeled him in to take the lead soon after the 35 kilometre mark.

The race was not his yet, though, and with just under two hours on the clock, Tsegay swept past as the youngster grabbed a bottle of water.

The pair battled it out for the lead for a kilometre or more but by the time the Bird's Nest stadium came into sight, Ghebreselassie was more than 12 seconds clear.

"I was very close to him and started to think about gold but at 38km I had stomach problems and had to slow down a bit," Tsegay said.

"Maybe it was because of the hot weather."

Handed his nation's flag just before he entered the arena, Ghebreselassie ran the final straight with it to claim Eritrea's second world championship medal after Zersenay Tadesse's 10,000m silver in 2009.

"Here it is very difficult, but we Eritreans never give up until the finish line," he said.

The Kenyans had a disappointing morning with world record holder Dennis Kimetto and previous world record holder Wilson Kipsang both failing to finish and Paris marathon champion Mark Korir crossing the line in 22nd place. (Reporting by Nick Mulvenney; Editing by Ian Ransom/Patrick Johnston)