The cop drama, starring Lennie James, ascended to 3.24m (13.7%) in the 9pm hour - its best audience since its launch in late June.
Meanwhile, the second live show of Andrew Lloyd Webber's Superstar dipped further to 2.37m (10.5%) between 9pm and 10.30pm, adding 115k (0.7%) on +1.
Elsewhere in the slot, BBC One's Turn Back Time interested 3.24m (13.7%), Gordon Behind Bars fell to 1.89m (8%) on Channel 4 (+1: 268k/1.5%), and CSI took 1.67m (7.4%) for Channel 5 (+1: 168k/0.9%).
Olympics comedy Twenty Twelve - airing immediately after Line of Duty on BBC Two - rallied 300k week-on-week to 1.51m (7.4%), while Big Brother attracted 1.14m (6.1%) to Channel 5.
Britain's Secret Treasures mustered 3.11m (15%) for ITV1 at 7.30pm (+1: 122k/0.6%) and Love Your Garden followed with 3.1m (14.5%) at 8pm (+1: 152k/0.6%) - both outperforming 9pm's Superstar.
Overall, BBC One won primetime with 20.4% over ITV1's 14.4% (+1: 0.6%). BBC Two claimed third place with 8%, ahead of Channel 4's 5.2% (+1: 0.7%) and Channel 5's 4.8% (+1: 0.3%).
Once again, Lewis topped the multichannel shows with 1.02m (4.5%) on ITV3 between 8pm and 10pm, while Geordie Shore plummeted to 340k (1.9%) on MTV.
BARB ratings data supplied by Attentional