Created by Oscar-winning screenwriter Julian Fellowes, the programme averaged 7.39m (28.9%) on ITV1 between 9pm and 10.30pm, while 295k (1.2%) watched on ITV1 HD.
Before Downton Abbey, The X Factor attracted a massive 11.87m (42%) on ITV1 between 7.45pm and 9pm, while 690k (2.4%) watched in HD.
Earlier on the channel, The Cube grabbed 4.99m (21.6%) from 6.45pm and 284k (1.2%) on ITV1 HD.
The return of Inspector George Gently mustered 5.21m (18.%) on BBC One from 8.30pm, while 157k (0.6%) watched the programme on BBC HD.
Countryfile grabbed 5.8m (26%) on BBC One from 6.30pm and 167k (0.7%) on BBC HD, before Antiques Roadshow took 6.06m (22.5%) and 169k (0.6%) in HD.
Over at Channel 4, Undercover Boss brought in 950k (3.3%) in the 8pm hour and 275k (1%) on timeshift, then a screening of Juno secured 1.96m (8.1%) from 9pm and 266k (1.7%) on Channel 4 +1.
A repeat of tribute programme Alex Higgins: People's Champion appealed to 1.06m (4.3%) on BBC Two in the 7pm hour, before extended coverage of Live Snooker took 1.55m (5.8%).
Due to the snooker coverage, Match Of The Day 2 switched to BBC One, where it pulled in 2.5m (17.8%) from 10.30pm.
On Five, screenings of Jackie Chan's First Strike and 300 grabbed respective audiences of 834k (3%) and 2.23m (10.1%) from 8pm.
Overall, ITV1 dominated primetime with 31.3% (HD: 1.6%) against BBC One's 20.2%. Five came in third with 6.3%, followed by Channel 4 with 5.5% (+1: 0.9%) and BBC Two with 5.4%.
Elsewhere, The Xtra Factor averaged 926k (3.4%) for ITV2 and 94k (4.5%) on timeshift.
BARB ratings data supplied by Attentional