Ashleigh Butler and her dog Pudsey went through to next Saturday's final as the public's top choice, along with Welsh choir Only Boys Aloud.
> 'Britain's Got Talent' first semi-final: Live blog
Speaking on ITV2 spinoff show Britain's Got More Talent, Walliams said: "How can you not like Pudsey? He was on his hind legs for a really long time."
"I think they 100% got through on talent," Holden responded. "This show is not about image, but Pudsey is the best looking dog we've ever had."
"I think Ashleigh and Pudsey are a funny act. They are not Frank Sinatra or the Beatles. They are novelty. But they are completely joyous," Walliams continued, adding that he would like to see the dog "lick Simon [Cowell's] face" in the final.
Cowell wrote on Twitter after the show: "Thought [eliminated contenders] The Mend were amazing and I really want that dog!"
Britain's Got Talent's live semi-finals continue tonight (Monday, May 7) at 7.30pm on ITV1.
Photo gallery - Britain's Got Talent first semi-final: