Joss Whedon's superhero ensemble pulls in £4.1 million to again hold off competition from American Pie: Reunion and secure top spot.
Tim Burton and Johnny Depp's Dark Shadows remake debuts at number three with a £2.4 million haul ahead of Zac Efron's The Lucky One and the 3D re-release of Beauty and the Beast.
Jason Statham's Safe and Salmon Fishing in the Yemen both slip a place to numbers six and seven respectively.
New release Piranha 3DD debuts at number eight with a total of £242,889. The horror sequel's opening haul falls far below the £1.33 million total its predecessor Piranha 3D earned in 2010.
The UK box office top ten in full:
1. (1) Avengers Assemble - £4,169,087
2. (2) American Pie: Reunion - £2,546,626
3. (-) Dark Shadows - £2,404,029
4. (3) The Lucky One - £431,576
5. (4) Beauty and the Beast - £422,924
6. (5) Safe - £298,201
7. (6) Salmon Fishing in the Yemen - £259,550
8. (-) Piranha 3DD - £242,889
9. (-) How I Spent My Summer Vacation - £152,728
10. (7) The Hunger Games - £143,542
Watch an exclusive clip from The Avengers below: