Nicola Sturgeon has told Sky News that she "got overexcited" after she was caught on camera celebrating Jo Swinson losing her seat to the SNP.
Ms Swinson, 39, has announced she is stepping down as Lib Dem leader after losing by just 149 votes to Amy Callaghan in East Dunbartonshire.
Ms Sturgeon was waiting to speak to Sky News when the result was read out - with the SNP leader cheering and celebrating her party's victory with energetic fist-pumps.
"Sorry, I got overexcited," she said.
Her response has been meet with fierce criticism.
Former Commons Speaker John Bercow - who was a guest presenter during Sky News' election coverage, told Ms Sturgeon that Jo Swinson's speech was "extremely gracious".
He said: "While acknowledging the enormous triumph of the SNP in East Dunbartonshire - which deserves to be acknowledged - I did think Jo Swinson made an extremely gracious concession speech on which she should be congratulated.
"It's a very difficult situation and I think she handled it to her enduring credit."
Nicola Sturgeon hit back, saying that she had already "conceded that" - and offered her "commiserations" to Jo Swinson.
She said: "On a personal level, I really do feel for her, but of course I can't help but be delighted by the performance of my party this evening."
Sky News Scotland bureau chief James Matthews says he's "never seen Nicola Sturgeon so animated".
He said: "It was everything short of a knee-slide across the media platform inside Glasgow's SEC - and a reflection of surprise within the SNP at just how well the party had done."
Brexit Party leader Nigel Farage has tweeted to claim her response was "graceless and nasty".
While Lib Dem MP Layla Moran - who held on her seat of Oxford West and Abingdon - said it was "so unacceptably ungracious".
Ms Sturgeon has claimed it was an "understandably, excited reaction" - and offered Ms Swinson her sympathy.
She said: "I understand more than most, the pressures and the challenges of leadership and to lose her seat tonight when she's lead her party through this campaign will be a bitter blow for her.
"It's difficult for women in politics, perhaps even more so for male leaders, so I have a great deal of sympathy for her - and I agree, I think the speech she made there was incredibly gracious."
Sir Ed Davey and Baroness Sal Brinton will replace Ms Swinson as joint acting leaders, until the Lib Dems hold a leadership election in the New Year.
The election is now officially over after the final count, in St Ives, was declared.
Conservative Derek Thomas held his seat after securing 25,365 votes - with a majority of 4,284.
The result confirms the Tories have an official majority of 80 in the House of Commons.