Brendan Rodgers' side maintained their impressive start to the season as they moved up to third after romping to a fourth win in their last five Premier League games, while Newcastle sit second bottom after winning just once in their opening seven matches.
Ricardo Pereira opened the scoring in the first half before Newcastle's Isaac Hayden was sent off for a dangerous tackle just before the break, and Leicester ran riot in the second half as Jamie Vardy struck either side of Paul Dummett's own goal.
Wilfred Ndidi rounded off Leicester's biggest league win since Rodgers arrived from Celtic to replace Claude Puel in February.
Bruce was furious with his players' second half surrender and made it clear he won't tolerate a repeat.
"It was totally unacceptable. The red card makes it difficult but you have to have more hunger. It was one of them horror shows," Bruce said.
"It was not anywhere near good enough. We have to show more than what we did and roll our sleeves up more in difficult circumstances.
"As soon as the third went in the heads went down. The lack of effort towards the end was nowhere near good enough. That is the difficult thing to take."
Mike Ashley's decision to hire Bruce from Sheffield Wednesday was greeted with disdain by Newcastle fans and the under-fire 58-year-old already appears to be fighting to save his job.
With Ashley still being linked with a deal to sell Newcastle, Bruce could be gone before a regime change or very soon after unless results improve quickly.
"The one thing I pride myself on is being honest. I have only been here a few weeks but you learn on performances like today," Bruce added.
"I always knew it was a challenge, I was delighted to take the challenge but let us hope we can turn it around. I have the appetite to do that."
Rodgers' appointment has proved an inspired move and Leicester look capable of challenging for a top four finish, especially given the inconsistent form of Manchester United, Arsenal, Tottenham and Chelsea.
"We've got a lot to learn still and a lot of improvements to make but the boys are absolutely brilliant," Rodgers said.
"Like I say they want to learn which, as a coach, it's the dream ticket really.
"Fantastic goals and a clean-sheet which is important because we want to defend well. Overall it was a great day for us."
- Newcastle in disarray -
Portugal right-back Pereira has earned a reputation for lung-bursting surges that can unhinge opposing defences and once again he was too hot for Newcastle to handle in the 16th minute.
Playing a one-two with Ayoze Perez, Pereira surged towards the Newcastle area unchecked by the visitors' defence before firing a fine finish into the bottom corner for his second goal in his last two games.
Bruce already cut a bedraggled figure on the touchline as the rain lashed down at the King Power Stadium.
And his day got even worse in the 43rd minute when Hayden was sent off for a reckless studs-up lunge on Praet that left the Leicester midfielder crumpled on the turf.
In the 54th minute, Dennis Praet picked out Harvey Barnes and cushioned a first-time pass over the top into the path of Vardy, whose low shot evaded Martin Dubravka's weak attempt to save at his near post.
Vardy's fourth goal of the season was followed by Leicester's third of the day three minutes later.
Praet advanced down the right and his shot was turned into his own net by Dummett.
With Newcastle in disarray, Vardy netted again after 64 minutes when he met Marc Albrighton's cross with a clinical header at the far post.
Leicester showed no mercy to Bruce and Ndidi turned swiftly to fire home in the 90th minute.