The competition at London Velopark got under way with two spectacular crashes on the challenging course. In a section made up of small jumps. American rider Brooke Crain clipped her rear wheel on a ramp and flew over the front of her handle bars. She slammed her chest into the next ramp and remained on the ground for about a minute.
Crain managed to get back on her bike to cross the finish line and was listed as DNF on the result sheet. If she's able to continue, she would be given the 16th and final seed in the women's competition.
Crain was a late replacement on the U.S. team for Arielle Martin, who was hospitalized after a crash during a final training run July 30 in California.
Latvian rider Edzus Treimanis then fell on the track at the same place. He went over his handlebars and landed face first. He knelt on the ground with blood running from his helmet, then took it off to reveal a cut to his forehead and he limped off.
Despite the crashes, British rider Liam Phillips was delighted by the course.
"It's fantastic, absolutely fantastic," he said. "It's truly Olympic standard."
A contender for the gold medal, Buchanan got off to a strong start in the tournament, clocking 38.434 seconds to edge former world champion Sarah Walker of New Zealand by 0.210 seconds. Mariana Pajon of Colombia was third in 38.787.
Both the men's and women's events start with a seeding phase to ensure that the fastest riders don't meet before the final. The women advance straight to the semifinals while the men's tournament features quarterfinals.
A silver medallist in Beijing four years ago, Laetitia Le Corguille of France ended fourth and will be in the same heat as Buchanan in Friday's semifinals.
The qualification run failed to spark excitement in the stands, in stark contrast to the overheated atmosphere in the nearby Velodrome.
British rider Shanaze Reade's ride was cheered by the crowd, although she only managed to post the fifth best time in 39.368.
Reade, a three-time world champion, was eliminated from the 2008 Olympic final after crashing in the final corner in an attempt to go for the gold medal.
Magalie Pottier of France, who won the world title in May, finished seventh.
In the men's competition, Van der Biezen clocked 37.779 to beat Beijing silver medallist Joris Daudet of France, who was 0.442 off the pace.
American Connor Fields, who won all three time trial superfinals of the 2012 world cup and looked poised to dominate the qualifying round, ended fourth in 38.431, while world champion Sam Willoughby of Australia ended sixth.
The Australian team said he won't speak to the media until Friday to keep his focus on his racing.
Tory Nyhaug of Coquitlam, B.C., finished 20th in the men's seeding run with a time of 39.515 seconds.
Willoughby will be in the same quarter-finals heat as defending Olympic champion Maris Strombergs, who had to be content with the 11th fastest time.
BMX was first included in the Olympic program at the Beijing Games and is generally a crowd-pleaser. After the seeding runs, the races are likely to be more spectacular with all riders starting together from the top of a big ramp.
The men's and women's course are slightly different, but both are technically challenging. The women's course is shorter and takes the riders through a tunnel before rejoining with the men's course on the last of the three banked corners. Instead of going into the tunnel, the men must negotiate a big jump on their second corner.