Half the 50 riders rode their dressage test that starts the three-phase competition which includes cross-country and show jumping. The remainder will ride on Sunday, including Zara Phillips, granddaughter of Queen Elizabeth II.
In the individual competition, Germany's Ingrid Klimke had a sparkling dressage test to score 39.3 penalty points, followed by teammate Dirk Schrade on King Artus with 39.8 and Mary King of Britain with 40.9 on Imperial Cavalier.
"I didn't think about the score," said Klimke. "That's up to the judges. My job was to ride a supple test."
Eventing standings are determined by lowest score. Each team includes up to five riders, with the combined scores of the three lowest riders to win team medals and the overall lowest riders to win individual honours.
The Greenwich Park stands hold seats for 23,000 for dressage and show jumping, while 50,000 spectators are expected on the cross-country course Monday. Medals in both team and individual competitions will be presented Tuesday after the show jumping phase.
The cheering for hometown heroes was upsetting to the British horses. King — a veteran of five Olympics and winner of two previous team medals — put her fingers to her lips as she emerged from the tunnel at the end of the ring and the anxious crowd responded with silence.
"It's absolutely fantastic here," King said after her ride as tears streaked down her face. "My horse has tremendous enthusiasm and the crowd helped me. I can't thank them enough."
Several teams, including favourites Britain and New Zealand, did not have a complete rotation of three riders Saturday, so team standings are still preliminary.
American riders included Boyd Martin of Cochranville, Pennslyvania, scoring 50.7 penalty points on Otis Barbotiere; Karen O'Connor of The Plains, Virginia, earning 48.2 on Mr. Medicott; and Tiana Coudray of Ojai, California, with 52.0 on Ringwood Magister.
Phillips is the daughter of former British eventing Olympians Princess Anne and Mark Phillips, who is coach of the American team here. He selected Martin to be the first U.S. rider to test the cross-country course Monday.
"It's a team competition, and Boyd is our best competitor," said Phillips.