Andy Ruiz Jr has caused one of the biggest shocks in the history of heavyweight boxing by stopping Anthony Joshua in seven rounds to inflict his first defeat and win the IBF, WBA and WBO titles.

On the occasion of the defending champion's US debut, at New York's revered Madison Square Garden, he was dethroned as the world's leading heavyweight by a significant underdog who took advantage of becoming a late-notice opponent to secure a life-changing win.

Brooklyn's Jarrell Miller had been chosen as Joshua's opponent until a six-month suspension ruled him out and gave Ruiz Jr an unexpected opportunity with little over a month's notice.

When he was knocked down in the third round, he appeared on the verge of losing as routinely as Joshua's previous 22 opponents, but he immediately responded with two knockdowns that left Joshua fortunate to survive.

If Joshua appeared to recover throughout the fourth and fifth, in the sixth he again became hesitant even if there was little sign that he was about to be stopped.

Two further knockdowns in the seventh again left him fighting for survival, and though he returned to his feet after each, he was no longer able to defend himself, forcing referee Mike Griffin's intervention while he was on his still on his feet after one minute and 27 seconds.

It was a straight right followed by a left hook that sent Ruiz Jr, who had been criticised for being fleshy and over 20lbs heavier than the champion at Friday's weigh-in, to the canvas in the third round.

Joshua had previously struggled with punching down against smaller opponents – his challenger had repeatedly fallen short with the jab because of the size disadvantage he faced – but there was no suggestion that he was about to suffer the most damaging round of his career.

When Ruiz Jr returned to his feet, Joshua immediately pursued the stoppage but Ruiz Jr fought to survive, landing a right flush on Joshua's chin, forcing the first heavy knockdown before another with two left hooks after Joshua had beat the count.

That dramatic round's end came with Joshua on the verge of a shock stoppage, but after spending much of the fourth and the fifth – in which he landed a powerful left hand – recovering, he struggled to build any momentum in the sixth.

Joshua became cautious in a way he never previously had against Dillian Whyte, who also once hurt him, and Wladimir Klitschko, who knocked him down, and Ruiz Jr capitalised by landing to the body.

A right hook then hurt Joshua again in the seventh before further lefts and rights forced him down, and as he returned to his feet, a further assault knocked him over as he again beat the count.

But he remained sufficiently unsteady that Griffin rightly rescued him from further punishment.

Joshua-Ruiz Jr had been intended to showcase the defending champion to the only territory in boxing he had yet to convince, yet if he again proved how entertaining he can be he suffered his first defeat, lost his three world titles and suffered significant damage to his reputation.

An immediate rematch may be inevitable, but at the venue where heavyweight greats Muhammad Ali, Joe Frazier, Lennox Lewis and Evander Holyfield enjoyed some of their finest nights, he suffered perhaps his worst as Mexico's first world heavyweight champion was instead crowned.

"I can't believe I just made my dream come true, the sky is the limit," said Ruiz Jr, like Joshua aged 29.

"That was the first time I've been down on the floor and it just made me stronger.

"I'm a Mexican warrior, I have that blood in me. I just made history for Mexico."

Joshua was booed by many of the spectators who cheered him to the ring, and he said: "Respect to Andy Ruiz Jr. Sorry I let my fans down."

"This will devastate him," said his promoter, Eddie Hearn.

"He will come back. It's now down to the rematch and winning that fight.

"He must win that fight and he will do anything he can to regain his title.

"To get back to the heights he has been, he must win that rematch."