(Adds Bieber apology, timings confusion)
By Mike Collett-White
LONDON, March 5 (Reuters) - Canadian singer Justin Bieber apologised on Tuesday after fans booed him for turning up nearly two hours late to a London concert, blaming technical issues for his late performance.
The 19-year-old teen idol had been scheduled to take the stage at London's O2 Arena at 8.30 p.m. but said in a Twitter message that the time was pushed back to 9.35 p.m. due to "some technical issues".
Many fans, and the venue itself, appeared not to have got the message after complaints poured in that Bieber was nearly two hours late when he finally appeared at around 10.20 p.m..
"Last night i was scheduled after 3 opening acts to go on stage at 935 not 830 but because of some technical issues," Bieber wrote on Twitter, the micro-blogging site where he boasts the largest following of more than 35 million people.
"i got on at 10:10..so...i was 40 min late to stage. there is no excuse for that and I apologize for anyone we upset. However it was great show and Im proud of that."
In the third of four tweets, he vowed to run on time on Tuesday, and in the final message said his relationship with the media, which picked up on fans' displeasure following Monday night's concert, was "not always easy but im trying".
The popular Sun tabloid newspaper said many fans, some of them as young as five years old, had gone home by the time Bieber began while others voiced frustration.
There was no on-stage apology, although the O2 Arena did later address fans, popularly known as "Beliebers", apologising in a Twitter message to "all the Justin Bieber fans for the lateness of his show tonight".
Bieber is due to play the same venue on Tuesday, Thursday and Friday.
Many people voiced their frustration at having to wait, while others reacted angrily to the headlines.
"Justin Bieber is my fave person but 2 hours late on stage is a joke!" fan Jess wrote on Twitter. "Does he realise that he has fans under the age of 10?"
Others jumped to his defence early on Tuesday.
"Feel really bad for @justinbieber now! Yes he was late but he put on a flipping good show! It was amazing," said one.
Not all reviewers were quite so kind for the singer who was named by Forbes magazine in 2012 as the third most powerful celebrity in the world.
London's Evening Standard awarded the "Baby" singer two stars, saying he turned "victory into defeat".
"By the end, the O2 was barely half-full and when Bieber asked 'Who's seen me play before?', he might have been better wondering who would spend time, money and adoration on seeing him again," wrote critic John Aizlewood.
Bieber's visit to Britain has probably not been his best.
On March 2, the day after he turned 19, he tweeted "worst birthday" amid reports some of his entourage were turned away from a London nightclub because they could not supply adequate proof of their age.
Bieber, who was discovered on YouTube in 2008, last month became the youngest artist to land five chart-topping albums in the United States following the release of his latest record, "Believe Acoustic". (Reporting by Mike Collett-White, Editing by Belinda Goldsmith)