Britain's Liam Phillips crashed and Shanaze Reade finished sixth as Maris Strombergs of Latvia and Mariana Pajon of Colombia won the men's and women's Olympic BMX titles at the London 2012 Velopark.
After Reade finished well behind, Phillips, who suffered a fractured collarbone at May's BMX World Championships in Birmingham, tumbled to the tarmac on the penultimate turn.
The 23-year-old cyclist from Burnham-on-Sea appeared to start well, but finished eighth as Strombergs successfully defended his title. World champion Sam Willoughby of Australia was second with Colombia's Carlos Oquendo third.
With Prime Minister David Cameron, his wife Samantha, Chancellor George Osborne and David Beckham all among the 6,000 crowd at the Velopark, Reade started poorly and could not recover as Pajon won, with Sarah Walker of New Zealand second and Laura Smulders of Holland third.
Reade crashed out of the 2008 Olympic final in attempting a gold-medal-or-bust manoeuvre and was among the favourites in London.
There was one run in the final and Reade was never in contention for the top place on the podium.