In the opening number, the five groups performed a medley of songs including One Direction's You Don't Know You're Beautiful and OutKast's Hey Ya.
Judge Martim Kemp walked on and opened the performance by shouting: "Lets kick tonight off with an A Capella battle!"
However, those watching at home were not pleased with what they heard.
One social media user commented: "That was hurting my ears that opening number #LetItShine."
Let It Shine viewers were less than impressed by the opening performance
Let It Shine: The boy bands performed a medley of hits A cappella
That opening was an absolute cringe fest
Let It Shine viewer
"Why are all the bands on #letitshine so flat!? Standard is terrible. Also, Ricki Lake? #current," another remarked.
Someone else tweeted: "Well that opening was an absolute cringe fest thanks BBC for wasting my licence fee #LetItShine #Refund."
A fourth simply said: "Grief! Beyond bad #letitshine."
Let It Shine fans complained that the standard was "terrible"
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But some viewers enjoyed the number, which gave each of Gary Barlow's 25 remaining hopefuls a chance to showcase their talents.
One fan said: "#LetItShine the groups opening the show were really good! Most talent shows are so bad now a days. I was shocked! Might start watching it!"
Another agreed, adding: "@GaryBarlow #LetItShine Oh Mr Barlow! What you've done! Such a great opening! They all will be massively famous!!"
Let It Shine continues next Saturday at 6.55pm on BBC One.