Britain's star-studded event, which featured appearances from Mr Bean and James Bond, shot to an astronomical high at 9.50pm on BBC One (26.9m) and in 3D on BBC HD (100k), Broadcast says.
Nearly 90% of the viewing audience stuck with the Ceremony as it reached its closing stages after midnight, signalling a record audience for that time of night.
Digital Spy's initial data from Attentional indicates that Danny Boyle's live spectacular averaged 23.02m (82.7%) between 9pm and 12.50am. The figures do not account for those who watched online, on Eurosport, Sky Sports or on big screens.
Overnight figures suggest that the Olympics Opening Ceremony may become one of the UK's most-watched broadcasts of all time when official BARB data is released in a week's time.
The Olympics countdown logged 10.08m (44.8%) on BBC One between 7pm and 9pm, while Olympic Breakfast had an impressive 1.71m (39%) between 6am and 9.15am.
BBC One was rewarded with a huge all-day share of 52.7% - five times more than its closest competitor ITV - as well as a primetime average of 65.8%.
Predictably, ratings on other channels suffered, with Big Brother falling to an all-time low of 614k (1.9%) on Channel 5 between 9pm and 10.30pm. The Bachelor followed with 242k (0.9%).
Meanwhile, The Million Pound Drop mustered 272k (3.1%) on Channel 4 and Gardeners' World secured BBC Two's highest rating with 1.26m (4.9%) at 8.30pm.
BARB ratings data supplied by Attentional
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