BEIJING (AFP) - Eritrea claimed the first gold of the world championships when unheralded teenager Ghirmay Ghebreslassie produced a shock victory in the men's marathon on Saturday.

The opening day of the worlds in Beijing will later see sprint rivals Usain Bolt and Justin Gatlin square up in the 100m heats while Mo Farah takes a shot at 10,000m history.

After a build-up marked by controversies over doping, competition finally got off to an early start when Uganda's Stephen Kiprotich defended his marathon title.

But in hot, clear conditions, he could finish only sixth as 19-year-old Ghebreslassie took the lead on the 36km mark. He saw off pressure from Ethiopian Yemane Tsegay for a memorable win in 2hr 12min 27sec.

"I am so proud of this victory. This is my first gold medal in my first marathon championship," said the Eritrean, crowned the youngest-ever world marathon winner in just his fourth race.

"It's a very special thing for me. It gives me motivations to continue to work hard. My medal is the first gold medal in Eritrean history."

Eritrea had previously only ever claimed one medal - a silver for Tadese Zersenay in the 10,000m in the 2009 worlds in Berlin.