The show, in which eight acts performed in front of a live studio audience for the chance to go through to Saturday's final, pulled in a 41.9% share of the audience for ITV1 between 9pm-10.30pm.
Viewers were able to vote for two acts and will be able to vote again this evening for two more acts during the second live semi-final.
BBC One's one-off 90-minute crime drama, 'Kiss of Death', drew 3.6m viewers and a 13.9% share between 9pm-10.30pm.
The ratings will disappoint the BBC, which had hopes for the Bank Holiday special because it was penned by the writer behind its popular crime drama 'Waking the Dead', which regularly pulls in more than 6m viewers.
A Channel 4 documentary, in which the programme makers used CGI to show what the planet will look like in the year 20100, was watched by 2m viewers and a 7.8% share between 9pm-10.35pm. Including Channel 4+1's time-shifted audience, 'Life After People', drew 2.2m viewers and an 8.6% share.
BBC Two aired the recent Hollywood remake of 'The Four Feathers', starring the late Heath Ledger as a man struggling to restore honour to his name, in the primetime slot. It pulled 2.1m viewers and an 8.9% share between 9pm-11.05pm.
Another Bank Holiday movie, 'The Mexican', with Brad Pitt and Julia Roberts, netted 1.3m viewers for BBC One and a 15.4% share between 10.50pm-12.45am.