Beijing: Britain's Mo Farah put a difficult few months behind him to retain his 10,000 metres title in some style as the opening day of the world championships finally shifted the focus from the doping crisis in athletics on Saturday.

Even with the superstars of the sport taking centre stage on the first of nine days of action at the Bird's Nest Stadium, however, there were some reminders of the troubles that have engulfed the sport over the last three weeks.

A few boos spilled down from the stands when convicted doper Justin Gatlin was introduced ahead of his 100 metres heat, which he won with the fastest time of the day to take a confident step towards Sunday's highly anticipated showdown with Usain Bolt.

Farah's brilliant victory was achieved despite almost being tripped up on the last lap and also by the disruption to his preparations for Beijing following doping allegations levelled at his coach Alberto Salazar. Salazar denies any wrongdoing.

"It hasn't been an easy year," said the Olympic champion, who will go for an unprecedented back-to-back 5,000-10,000 metres world championship double next Saturday.

"As an athlete, I let my running do my talking. I've just got to keep doing what I'm good at and that is running and winning medals for my country."

Farah was almost tripped by Kenyan Geoffrey Kamworor as the leaders rounded the first turn after the bell but somehow stayed on his feet to deliver his trademark finish over the final straight and cross the line in 27 minutes 01.13 seconds.

"I nearly went down, but I managed to stay on my feet and win the race," said Farah. "The (Kenyans) worked as a team, so hats off to them, they tried something different and they really made it hard for me."

It was Farah's sixth straight success in major championship distance finals since his second place in the 10,000m at the 2011 worlds in Daegu.

The International Association of Athletics Federations (IAAF) would have been hoping for athletic excellence to take the spotlight after weeks of embarrassing leaks and accusations that it has neglected its duty to root out drug cheats.

They got it in the first event of the championships when Eritrean teenager Ghirmay Ghebreselassie became the youngest ever marathon world champion with a stunning run in punishing humidity.

The second gold medal was won when Germany's Christina Schwanitz clinched the women's shot put title by just seven centimetres from China's Gong Lijiao.

The 29-year-old disappointed the home crowd at the Bird's Nest stadium by edging Gong Lijiao into second place with her third throw, leaving the Chinese to take silver on the back of her opening effort of 20.30m.

American Michelle Carter threw 19.76m to beat Anita Marton, who bettered Hungary's national record with her best attempt of 19.48m, and take the bronze medal.

New Zealander Valerie Adams, who won gold at the last four world championships, elected to skip the meeting so she could undergo shoulder and elbow surgeries with an eye on her Olympic title defence in Rio de Janeiro next year.

"I must be one of the happiest people in the stadium," a laughing Schwanitz said, the German flag draped over her shoulders.

"The feeling is indescribable. It's unreal. I couldn't really see how I would do it."

Asked how she managed to surpass Gong's throw by seven centimetres, Schwanitz quipped: "Seven centimetres, that's hard core!

"I really concentrated, really focused. I knew I wasn't as nervous as she was and I know that in the world championships nothing is impossible.

"I was in a more relaxed position than I had ever been in before.

"I'll probably have a few beers with my trainer tonight."

That result may have disappointed a healthy crowd at the venue for the 2008 Olympics but the few Kenyans in the crowd would have felt harder done by.

A silver and bronze behind Farah for Kamworor and Paul Tanui came after two of the African powerhouse's vaunted trio of marathon runners failed to finish and the other came home 22nd.

Ghebreselassie crossed the line in two hours, 12 minutes and 27 seconds to claim a first world championships gold medal for his country.

"My parents wanted me to be a great student, but I wanted to be a great athlete," said the 19-year-old."Today's victory will be a great surprise to them."

"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.

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.

There was better news for Kenya in the 800m heats, where David Rudisha looked in good, if not dominating, form with a run of one minute 48.31 seconds to reach Sunday's semi-finals.

Another East African world record holder, Ethiopian Genzebe Dibaba, was in blistering form in the heats of the women's 1,500m, clocking four minutes 02.59 seconds for the fastest time ever in the first round at the world championships.

Britain looks like quickly building on Farah's golden start with Olympic champion Jessica Ennis-Hill and team mate Katarina Johnson-Thompson first and second after four of the seven events in the heptathlon. (Reuters)