Having been in the city all week - even helping switch on Regent Street's Christmas lights display - Clarkson's urge to perform got the better of her, with the last-minute decision being made to stage the event on her final night in the capital.
With just 200 fans at The Borderline, the 29-year-old opened her 90-minute set with a performance of 'Dark Side', taken from her new album Stronger. Wearing a bandage on her hand, Clarkson explained that she had badly burnt it on a takeaway cup of tea earlier in the day.
Among the tracks performed were hits 'Since U Been Gone', 'Walk Away', 'Never Again', 'Breakaway', 'Behind These Hazel Eyes', 'Miss Independent' and current single 'Mr Know It All'.
The Texan, who revealed that her mother was watching, wouldn't show off her 13th tattoo, which she had inked earlier in the day. To mark the release of her album, Clarkson revealed that her latest inking was morse code for 'What Doesn't Kill You'.
She went on to perform the song, which serves as the second single from Stronger, revealing that its video - which will have "worldwide" fan involvement - would be filmed soon.
Sipping vodka, Clarkson also performed fan favourite 'Sober', taken from her 2007 LP My December. Among other songs requested by fans were 'Chivas', 'Because of You', 'Gone' and a short, acapella version of new album track 'I Forgive You'.
Alongside a cover of 'Seven Nation Army', she also performed an impressive version of fellow American Idol champion Carrie Underwood's 'I Know You Won't'.
Earlier this week, the singer told Digital Spy how much she enjoyed performing in small venues, but joked that her label did not like the gigs as they made no money.
However, before closing the set, Clarkson thanked Sony UK for allowing her to perform the show. Wearing Union Jack sunglasses for her final performance, she ended with an energetic rendition of her number one hit 'My Life Would Suck Without You'.
Kelly Clarkson's fifth studio album Stronger is out now.
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