Justin Bieber unleashes his inner rockstar in Metallica T-shirt as he gatecrashes Jack U's headline set at summer festival

He's put a lot of effort into distancing himself from his teeny-bopper years.

And Justin Bieber took his efforts to the next level when he unleashed his inner rock star during his performance with Jack U at the Hard Summer Music Festival in Pomona, California, on Sunday.

The 21-year-old worked a new, grungy look as he hit the stage with US DJ duo Diplo and Skrillex for their headlining set in a grey Metallica T-shirt.

Scroll down for video 

New look: Justin Bieber unleashed his inner rock star when he performed with Jack U at the Hard Summer Music Festival in Pomona, California, on Sunday

New look: Justin Bieber unleashed his inner rock star when he performed with Jack U at the Hard Summer Music Festival in Pomona, California, on Sunday

The graphic tee was approximately four sizes too big and had the ...And Justice For All lyrics 'their money tips her scales again' scrawled in white print across the bottom.

But Bieber didn't completely do away with his usual style for his guest appearance. 

The Canadian hit-maker stuck with his trusty reverse snap back which kept his famous quiff in place as he cut some shapes on stage.

He also wore a pair of low-slung trousers and trainers - perfect attire for his energetic performance. 

Grimy: The 21-year-old donned his finest Metallica T-shirt as he worked the stage with US DJ duo Diplo and Skrillex during their headlining set

Grimy: The 21-year-old donned his finest Metallica T-shirt as he worked the stage with US DJ duo Diplo and Skrillex during their headlining set

So edgy: The graphic tee was about four sizes too big and had the ...And Justice For All lyrics 'their money tips her scales again' scrawled in white ink across the bottom

So edgy: The graphic tee was about four sizes too big and had the ...And Justice For All lyrics 'their money tips her scales again' scrawled in white ink across the bottom

Energetic: The Canadian hit-maker stuck with his trusty reverse snap back which kept his famous quiff in place as he cut some shapes on stage

Energetic: The Canadian hit-maker stuck with his trusty reverse snap back which kept his famous quiff in place as he cut some shapes on stage

The real deal: Bieber clearly looks up to Metallica's James Hetfield (left) and Kirk Hammett (right)

The real deal: Bieber clearly looks up to Metallica's James Hetfield (left) and Kirk Hammett (right)

Bieber collaborated with Jack U on their track Where Are U Now and was joined be a slew of backing dancers when they performed to the crowd. 

After the set Bieber took to Twitter to reach out to his friends and thank their fans for turning out to see them all.

He tweeted: 'Good times boys. Thank you La. @Skrillex @diplo #hardfest.'

Team effort: Bieber collaborated with Jack U on their track Where Are U Now and was joined be a slew of backing dancers when they performed to the crowd

Team effort: Bieber collaborated with Jack U on their track Where Are U Now and was joined be a slew of backing dancers when they performed to the crowd

All the rage: US DJ duo Diplo and Skrillex go by the name of Jack U when they unite  on stage

All the rage: US DJ duo Diplo and Skrillex go by the name of Jack U when they unite on stage

Appreciative: After their set Bieber took to Twitter to reach out to his friends and thank their fans for turning out to see them

Appreciative: After their set Bieber took to Twitter to reach out to his friends and thank their fans for turning out to see them

Meanwhile, Bieber admitted to journalists that the media furore surrounding the shooting of Cecil the lion in Zimbabwe had passed him by.

And although he wasn't familiar with the details of Cecil's death he said he was 'definitely against' killing animals for sport.

He told the press: 'I think that we should not kill for sport at all. You can kill to eat, but not like lions and stuff, obviously.

The last to know: Bieber recently admitted to journalists that the media furore surrounding the shooting of Cecil the lion in Zimbabwe had passed him by

The last to know: Bieber recently admitted to journalists that the media furore surrounding the shooting of Cecil the lion in Zimbabwe had passed him by

Standing strong: Bieber doesn't agree with shouting animals for sport  

Standing strong: Bieber doesn't agree with shouting animals for sport  

'I think if we could just raise as much awareness as we can for that,' he added.

Cecil was shot and killed on July 1 by US dentist Walter Palmer, 55, who had paid locals $50,000 (£32,000) for the privilege. Mr Palmer wounded him with a bow and arrow after luring him from his habitat in the Hwange National Park in Matabeleland North.

However, Cecil didn't die from the wound and was stalked for another 40 hours before he was eventually shot dead with a rifle. He was then skinned and his head was removed. 

Local legend: Cecil the lion was killed by American dentist Walter Palmer with a bow and arrow during an illegal hunt in Zimbabwe. Palmer, 55, from Minnesota, allegedly paid park guides $50,000 to hunt and kill Cecil at the Hwange National Park, not realising that the animal was a local favorite

Local legend: Cecil the lion was killed by American dentist Walter Palmer with a bow and arrow during an illegal hunt in Zimbabwe. Palmer, 55, from Minnesota, allegedly paid park guides $50,000 to hunt and kill Cecil at the Hwange National Park, not realising that the animal was a local favorite

The comments below have not been moderated.

The views expressed in the contents above are those of our users and do not necessarily reflect the views of MailOnline.

By posting your comment you agree to our house rules.

Who is this week's top commenter? Find out now