De Bruyne, along with fellow substitute Abdukodir Khusanov, netted after the break to cancel out Stefan Ortega's own-goal after Jamie Donley's stunning effort.

Charlie Kelman registered Orient's first attempt on target in the 14th minute, showing great pace to speed past City debutant Vitor Reis, but seeing his effort smothered by Ortega.

But two minutes later, Orient edged in front in sensational fashion when Donley's audacious effort from distance hit the underside of the crossbar, and bounced in off Ortega and into the back of the net.

Donley was then the hero at the other end, brilliantly clearing Bernardo Silva's strike off the line after the Portugal winger was picked out at the back post by Jack Grealish's dinked cross.

The Citizens were inches from finding a leveller in first-half stoppage-time, but Omar Marmoush's close-range volley was kept out by an excellent Josh Keeley save.

But Pep Guardiola's side drew level 11 minutes after the break when Rico Lewis' strike from the edge of the box deflected past Keeley, via a touch from substitute Khusanov.

The visitors continued to turn the screw in their quest for a winner, with James McAtee clipping the post before Ilkay Gundogan forced Keeley into a smart save down low.

However, with 11 minutes left, substitute De Bruyne sealed City's passage to the fifth round, prodding the ball home after collecting Grealish's pass in behind the Orient defence.

Orient went agonisingly close to sending the game to extra-time, but captain Dan Happe could only fire over Ortega's crossbar from Tom James's inviting free-kick.

City boss Pep Guardiola said City's scare was an example of the magic that makes the competition "unbelievable".

"We played a really good game, it was a typical FA Cup game – that's why this competition is unbelievable," Guardiola said.

"We knew that they were strong, and they'd run in the channels. They are a superb team and incredibly well managed, but we created a lot of chances. I'm happy to go through."

Guardiola was asked whether he was annoyed at having to rely on De Bruyne's quality in reserve to battle past the EFL League One side.

"Absolutely not, that is why they are here. We take this competition incredibly seriously, and we thought it was the best decision to make," Guardiola added.

"I am happy about Kevin's goal, his movement was really good."

Rico Lewis, whose shot deflected off Khusanov for the equaliser, believes that with Guardiola at the helm, City can still have a special season.

"He [Pep Guardiola] wasn't going mad or anything like that. He knew we'd created chances, and it's about keeping going in an environment like this," Lewis said.

"Credit to Leyton Orient, they made it difficult for us to play. It's easy to say it's tougher than you think, but we all know they're tough. The crowd are right up for it.

"It's Man City at the end of the day, the demands are so high. There's enough leadership in this team. Going from attack to defence is difficult, there's a lot of ground to cover.

"The season hasn't panned out how we wanted it to. Every game now, as long as we win, it could be a great season for us.

"Everyone wants us to get beat, apart from City fans, of course. It's pressure on our shoulders. Everyone settled their heads at half-time."