* Joshua knocks down former champion twice in 11th round
* Klitschko takes Briton beyond seventh round for first time
* Fight attracts biggest crowd in Britain for 78 years (Adds details and quotes)
By Ian Chadband
LONDON, April 29 (Reuters) - Anthony Joshua delivered one of the great nights in British boxing annals by stopping Ukrainian Wladimir Klitschko in the 11th round to be crowned IBF, WBA and IBO world heavyweight champion in front of 90,000 fans at Wembley Stadium on Saturday.
Britain's unbeaten IBF title holder earned a sensational victory by knocking down the 41-year-old former champion twice in the 11th and penultimate round before the referee stepped in to save Klitschko from any more punishment.
What was hailed as the biggest fight night ever staged in a British ring, watched by the largest crowd for a boxing show in Britain for 78 years, lived up to its billing.
It was a thrilling contest which saw both combatants clamber off the canvas seemingly on the verge of defeat and looks destined to be recalled as one of the great heavyweight title fights.
Joshua survived a knockdown for the first time in his professional career in the sixth round and looked close to surrendering his unbeaten record until his late bombardment forced the stoppage.
In a sensational fifth round, Joshua knocked down Klitschko only to end up hanging on desperately at the end of the round as the Ukrainian launched a remarkable comeback.
The veteran had even looked the more likely winner as he defied a 14-year age gap and was outboxing Joshua in the latter stages until the Briton produced a blistering finish to take his unbeaten record to 19 straight stoppage wins.
Both men had to dig deep and both looked close to exhaustion before the 27-year-old Joshua's youth, fitness and sheer power took over in a penultimate round that sent the huge crowd into ecstasy as two barrages sent Klitschko down.
"What can I say? 19-0, three-and-a-half years in the game. As I said, I'm not perfect but I'm trying," Joshua told the cheering crowd from the ring.
"As boxing states, you leave your ego at the door and you respect your opponent. So a massive shout out to Wladimir Klitschko."
"The best man won tonight and it's a massive event for boxing," responded Klitschko after his second defeat in succession at the hands of a British heavyweight following the loss of his titles to Tyson Fury 17 months ago after an 11-year reign.
"Two gentleman fought each other. Anthony was better today. It's really sad I didn't make it."
The fight attracted a gate that had not been matched for a British show since Len Harvey fought Jock McAvoy for the British light-heavyweight title at another London venue, White City, in 1939.
The mutual praise between Joshua and Klitschko echoed the civilised and respectful way the two former Olympic champions had behaved in the build-up to the contest but there was nothing civil about the brutal punishment they dished out to each other.
After four rounds of feeling each other out, with Klitschko's movement and Joshua's power quite apparent, the crowd were not prepared for an astonishing fifth round.
First, Joshua launched a blistering left hook and followed up with a flurry of punches that saw the Ukrainian drop to his knees and, when he rose groggily, take a standing count.
Klitschko suddenly looked old and the end seemed nigh as Joshua roared in to finish the job but that was when he found his champion's spirit in his desperation, landing a big left of his own to leave Joshua in real peril.
The Ukrainian continued in the sixth, setting up his opponent with the jab before a huge right cross sent Joshua down.
He scrambled off his knees but it did not look as if he would see the eighth round, uncharted territory for the Briton who had won all his previous fights within seven.
Yet after Klitschko had, remarkably, looked almost the younger of the two fighters in the stretch, Joshua demonstrated real heart to go with his power as he unleashed a right uppercut that signalled the final assaults on the Ukrainian's scrambled senses. (Editing by Clare Fallon)