More than 10 million people saw Rebecca Adlington's bronze medal-winning performance for Team GB.
Overnight viewing figures also showed 7.6 million tuned in to see Lizzie Armitstead's rain-sodden finish on The Mall as she became the first British athlete to make the medals table for London 2012.
Adlington added to her two 2008 gold medals when she finished third in the 400m freestyle final last night, with 10.1 million watching on BBC1 and 600,000 watching on one of the BBC's dedicated Olympics channels.
Earlier in the day, Armitstead collected the silver in the women's cycling road race.
Red-faced BBC Olympics boss Roger Mosey apologised after he confused Adlington's medal in a message on Twitter as he hailed the viewing figures.
He told followers Adlington had won a gold, correcting himself a little later by saying: "Sorry, I do of course mean bronze for Rebecca."
The figures also show the impact the BBC's domination of the sporting spectacle is having on viewing figures for other channels.
ITV1 - usually the main rival to BBC1 - had an all-day average of just 680,000 viewers on Saturday, the first day of competition. It drew just 6.2% of viewers during the day, thought to have been one of the channel's lowest ever shares.
It was only marginally ahead of the digital channel BBC3 - which is being used as a sports channel for the duration of the games - which was only 30,000 viewers behind.
The latest figures come on the heels of the huge audience for the opening ceremony on Friday night which drew 26.9 million viewers at its peak.
The average audience for the ceremony was 22.4 million, the highest on BBC1 since 1996.