The singer zoomed into the lead on the tally, while Little Mix currently hold second place with 'DNA'.
Meanwhile, Gabrielle Aplin's 'The Power Of Love' - which features on the John Lewis Christmas advert - is at number three, and One Direction's 'Little Things' completes the top five.
Further down the chart, McFly's 'Love Is Easy' is at six, Stooshe's cover of TLC's 'Waterfalls' sits at 18 and Ed Sheeran's 'Give Me Love' is at 24.
Pink's 'Try' has scaled the tally to number 33, Example's 'Close Enemies' stands at 43, and Lana Del Rey's 'Ride' starts at 55.
On the album chart, One Direction occupy spots one and two with the separate editions of their new LP Take Me Home, while Del Rey's Born To Die: The Paradise Edition is at three.
Elsewhere, The Rolling Stones' GRRR! is currently at four, Green Day are at seven with Dos!, and Christina Aguilera's Lotus is at 13.
The official singles and album charts are announced on Sunday between 4pm and 7pm.
Watch the video for Mars's 'Locked out of Heaven' below: