The new retelling of Arthurian legend focuses on the wizard's uncharted younger days and stars Richard Wilson, Anthony Head and John Hurt. The 13-episode first series has also been bought by NBC in the US, where it will air from next January.
The first episode had an average of 6.65m (30%) over 45 minutes from 7.30pm, while on ITV1, ratings juggernaut The X Factor brought in 9.26m (41.7%) over an hour. The audience for Merlin was steady throughout but grew towards the end with a peak of 6.82m (30.3%) at 8.10pm.
Before Merlin, the new series of Strictly Come Dancing launched with 7.99m (42.7%) over 80 minutes from 6.10pm. The audience climbed steadily from 6.41m (39%) at 6.15pm to a peak of 9.61m (45.6%) by the end. On ITV1 from 6.30pm, countdown special You've Been Framed! Funniest 100 had 4.13m (21.3%).
After The X Factor, All Star Family Fortunes was seen by 5.25m (24.4%) until 9.15pm. It beat BBC One's The National Lottery: Who Dares Wins, which averaged 4.86m (22.4%) between 8.15pm and 9.05pm.
Channel 4's screening of the second Lord of the Rings movie, The Two Towers, held a respectable 1.77m (8.7%) between 7.30pm and 10.55pm, keeping the channel ahead of BBC Two for most of the evening.
BBC Two peaked with 1.62m (7.5%) for Christina: A Medieval Life at 8.30pm. First shown on BBC Four, the documentary told the story of a fictional peasant family living in the 14th century. Earlier, a repeat of Stephen Fry and the Gutenberg Press interested 941,000 (4.2%). At 9.30pm, Have I Got Old News For You averaged 1.41m (6.8%) and Mock The Week had 1.18m (6.4%).
Five started with 979,000 (4.5%) for NCIS at 8pm. A second episode, shown at 8.55pm, brought in the channel's all-day high of 1.36m (6.4%). CSI had 1.05m (6.1%) at 9.55pm then Law & Order: Criminal Intent notched up 926,000 (8.3%).
The Xtra Factor was the best performing multichannel programme, with 1.08m (5.7%) for ITV2 from 8.30pm. A further 111,000 (0.6%) watched on ITV2+1.
BBC Three held a strong 649,000 (4.8%) from 9.30pm for its two-and-a-half hour countdown show The Most Annoying Pop Songs We Hate To Love.
Elsewhere, a QI marathon proved popular for Dave in primetime, with a peak audience of 395,000 (3.7%) for its 11pm episode.
> Click here to read our review of Merlin episode one