Starring Jennifer Saunders and Joanna Lumley, the revived BBC One sitcom attracted 5.61m (24.1%) at 9.30pm.
Celebrating its 20th anniversary, Ab Fab's previous Christmas and New Year's specials pulled in 7.4m and 7.1m respectively.
Olympics-themed programming aired elsewhere in BBC One's schedule, with a one-off Question of Sport scoring 3.53m (15.4%) at 8.30pm, and late-night Tom Daley documentary Diving for Britain striking gold with 2.89m (20.6%).
EastEnders provided the channel's most-watched broadcasts with 7.6m and 7.3m for its double bill, which contained live scenes.
Despite airing 90 minutes of soaps, ITV played second fiddle to BBC One in primetime, as Superstar drew a poor 2.59m (11.3%) from 9pm (+1: 145k/0.8%), and 2.51m (11.6%) caught Countrywise at 8pm.
Ruby Wax's Mad Confessions launched on Channel 4 with 730k (4%) at 10pm (+1: 125k/1.2%), while Location, Location, Location returned with 1.23m (5.5%) at 8pm (+1: 209k/0.9%) and Undercover Boss had 1.34m (5.7%) at 9pm (+1: 219k/1.2%).
Meanwhile, Big Brother underperformed with 995k (4.2%) for Channel 5 at 9pm (+1: 149k/0.8%), then The Walking Dead took 695k (3.7%) at 10pm (+1: 124k/1.1%). Megastructures interested 669k (3%) earlier on.
Overall, BBC One secured a rare Monday night victory with 22.9% versus ITV1's 19.8% (+1: 0.7%). BBC Two claimed third place with 6.9%, followed by Channel 4 with 4.8% (+1: 0.9%), and Channel 5 with 3.4% (+1: 0.3%).
BARB ratings data supplied by Attentional