Further back there was disappointment for Britain's Paula Radcliffe, who won the 2005 world title, as she faded to finish 23rd.
|Briton Paula Radcliffe's Beijing dreams were shattered [GALLO/GETTY]
She finished the race in tears and limping badly on the left leg which had suffered a stress fracture earlier this year.
"Perhaps I pushed too hard to come and compete here," the 34-year-old Radcliffe said afterwards.
America's 2006 London Marathon champion Deena Kastor, one of the favourites for the title and the bronze medalist from Athens, also faired badly, dropping out shortly after the 5km mark.
"My foot has been sore for the past week," said Kastor. "It was tight most of the way. I felt a pop in my foot and I couldn't stand on it."
Earlier the 80 competing runners had set out at a steady pace from Tiananmen Square in central Beijing, helped by relatively cool temperatures and low humidity.
Tiny Lesotho runner Mamorallo Tjoka had a good first 10 km, but suffered a grievous blow just after that when her right ankle went and she had to pull to the side of the road.
She managed to get going again and though clearly in some agony she regained the lead before finally giving up shortly after the 15 km mark.
As the pack headed past the 20 km mark, Tomescu pulled ahead with an extra burst of acceleration, giving her a four second lead by the halfway point which grew and grew.
By the time she entered the Bird's Nest she had expanded that lead to nearly a minute, as she sprinted the final leg to take gold.