The hard-hitting thriller, starring Lennie James, launched with 3.35m (14.7%) on BBC Two and BBC HD.
Ranking as BBC Two's best-rated drama in 2012 to date, Line of Duty retained a steady audience from start to finish.
With viewers spread fairly evenly across the terrestrial channels, BBC One came second in the slot with 3.17m (13.9%) for new factual experiment series Turn Back Time: The Family.
Meanwhile, the concluding part of ITV1's Strictly Kosher - a programme following a Jewish community in Manchester - interested 2.36m (10.3%) and 210k (1.2%) on +1.
Gordon Ramsay's new Channel 4 cooking-in-prison vehicle Gordon Behind Bars beat ITV with 2.64m (11.6%) - adding 321k (1.8%) on timeshift.
Import CSI was last in the slot with 1.55m (6.8%) and 171k (0.9%) on +1, but Channel 5 had a solid night overall with 8pm's Killers Behind Bars (1.32m/6.1%) and 10pm's Big Brother (1.28m/7.2%).
Earlier in the 8pm hour, an hour-long Love Your Garden beat ITV1's usual slot average with 3.08m (14.2%) and 121k (0.5%) on timeshift, while Embarrassing Bodies grabbed 1.6m (7.4%) for Channel 4 (+1: 336k/1.5%), and Today at Wimbledon took 1.44m (6.7%) for BBC Two.
BBC Two's peak time share was boosted by early evening coverage of Wimbledon, which had 2.73m (14.4%) between 5.50pm and 8pm for Andy Murray's first round win.
Overall, BBC One topped primetime mostly thanks to the popularity of EastEnders and Holby City. ITV1 followed in second place with 13.5% (+1: 0.7%), ahead of BBC Two's 10.8%, Channel 4's 7.3% (+1: 1%), and Channel 5's 5.5% (+1: 0.3%).
