Michael Phelps became the most decorated Olympian of all time, winning a record 19th medal in the pool as China's swimming sensation Ye Shiwen grabbed her second gold of the Games.
Phelps anchored the US team to a resounding victory in the 4x200m freestyle relay to secure gold and beat the record of 18 medals amassed by Soviet gymnast Larisa Latynina between 1956 and 1964.
The American had earlier been thwarted in his bid for a 200m butterfly Olympic treble by Chad le Clos of South Africa, but a silver medal allowed him to match the record medal haul before the relay sealed his place in history.
Phelps, 27, won six golds and two bronze medals at the 2004 Athens Games, then collected a record eight golds in Beijing.
He had suffered a testing start to the London Games, slumping out of the medals in the 400m individual medley and then having to settle for silver in the 4x100m freestyle relay.
But Phelps' unprecedented 19th medal was never in doubt in Tuesday's relay as he took over the final leg with a clear lead built up by his team-mates and powered home to leave France trailing in silver and China with bronze.
"I thanked those guys for helping me get to this moment," said Phelps. "I told those guys I wanted a big lead in the last leg and they gave it to me.
"I just wanted to hold on."
Ye, just 16, may encounter more speculation about her remarkable performances after she added the 200m individual medley title to the 400m individual medley crown.
The Chinese girl was lagging behind the leaders at the turn in the Aquatics Centre, but produced a powerful final 50m to overhaul Alicia Coutts of Australia in an Olympic record time of 2min 7.57sec.
Her swim came just days after her lightning freestyle leg in the longer medley propelled her to a new world record.
Ye's feats have raised unproven suspicions of doping, with John Leonard, executive director of the World Swimming Coaches' Association, describing her finish in the 400m medley as "impossible".
But Ye said she had used the row to motivate herself.
"I don't feel upset or sad about what the media have been saying about me," she insisted. "I feel calm, but it just encouraged me to prove myself."
Chinese authorities went on the offensive over the allegations. Jiang Zhixue, the head of anti-doping at China's General Administration of Sport, told Xinhua news agency it was unfair to point the finger at their swimmers.
"I think it is not proper to single Chinese swimmers out once they produce good results. Some people are just biased," he said.
Former British swimmer Adrian Moorhouse, a gold medallist in the 100m breaststroke in the 1988 Seoul Games, defended Ye, saying it was possible China had unearthed their own Phelps.
"I think it's sour grapes," Moorhouse said. "I think it's quite insulting actually."
In the gymnastics arena, the United States won the women's team final to claim their first gold medal in the event since 1996. Russia had to settle for silver while Romania pipped 2008 Olympic champions China to bronze.
Defending champions Germany won the equestrian three-day eventing, with Michael Jung then becoming the first rider to hold the European, world and Olympic individual titles.
Great Britain finished second in the team event to earn a silver medal for Queen Elizabeth II's granddaughter Zara Phillips, but the host nation is still without a single gold medal after four days of competition.
The US basketball Dream Team thrashed Tunisia 110-63 as Carmelo Anthony and Kevin Love each scored 16 points, leaving the Americans heading for a date with Nigeria and eyeing the next round.
China lead the United States by 13 golds to nine in the medals table, and the Chinese won their third diving title in three events when world champions Chen Ruolin and Wang Hao clinched the women's 10m synchro platform.
Chen, the individual 10m platform champion from the Beijing Games and the synchronised 10m platform champion, teamed with Wang to tally 368.40 points from their five dives off the tower.