LOS ANGELES (Reuters) - Pop star Justin Bieber struck back at his critics on Thursday, denouncing the "countless lies" written about him after a week in which he canceled a concert, collapsed on stage and had a run-in with paparazzi.

In a lengthy posting on his Instagram profile, the 19-year-old singer said he was not going into rehab, contrary to some media speculation, and felt he had done nothing to deserve the negative press surrounding his European tour.

"Everyone in my team has been telling me, 'keep the press happy' but I'm tired of all the countless lies in the press right now. Saying I'm going to rehab and how my family is disappointed in me," Bieber wrote.

Saying he wanted to let his fans know directly how he felt, Bieber added: "I'm a good person with a big heart. And don't think I deserve all this negative press I've worked my ass off to get where I am and my hard work doesn't stop here ... All this isn't easy. I get angry sometimes. I'm human. I'm gonna make mistakes. I'm gonna grow and get better from them."

The clean-cut Canadian teenager has had a meteoric career since being discovered on YouTube in 2008.

But he hit the headlines for all the wrong reasons last week when he turned up two hours late for a London concert, collapsed on stage in London with shortness of breath on another night, and was caught on camera cursing at a pushy photographer.

He was spotted wearing a bizarre gas mask on a night out in London, and one British tabloid labeled him "Pop brat Justin."

On Monday, he canceled one of two planned concerts in Portugal this week. A statement cited "unforeseen circumstances," but local media in Portugal reported that ticket sales for the gigs had been slow.

Bieber, who has more than 35 million Twitter followers, suggested on Thursday that envy of his success might lie at the root of the jibes from the news media.

"I'm 19 with 5 number one albums, 19 and I've seen the whole world. 19 and I've accomplished more than I could've ever dreamed of, I'm 19 and it must be scary to some people to think that this is just the beginning," he wrote.

Bieber played a concert, apparently without incident, in Madrid on Thursday before moving on to Barcelona and other European cities later this month.