Alfred -- who had never finished on the podium at a major outdoor championships prior to Saturday's final -- conjured a superb race to win in 10.72sec as the USA's Richardson took silver in 10.87sec and compatriot Melissa Jefferson won bronze in 10.92sec.

Richardson, the reigning world champion over 100m, had been hotly tipped to become the first American to win the coveted Olympic crown since Gail Devers in 1996.

But the 24-year-old Texan found herself outgunned by the powerfully built Alfred, who led from start to finish after exploding out of the blocks at a rainswept Stade de France.

Alfred's gold gave St. Lucia their first ever medal of any colour at an Olympics. The island nation only began competing at the Games in 1996.

It was an incredible performance by Alfred who had fired a warning shot across Richardson's bows in the semi-finals, finishing ahead of the American in 10.89sec.

The semi-final proved to be an ominous portent for Richardson, who was slow out of the blocks and never looked like reeling in Alfred who could even afford to ease up as she crossed the line to clinch gold.

Alfred was in tears of celebration after sealing a sensational win, running to ring the trackside victory bell before wrapping herself in her country's flag and embracing her American rivals.