How do you get 15,000 fans at a Taylor Swift concert to yell at the top of their lungs for minutes on end? Bring out teen singing sensation Justin Bieber to perform a duet with the country star.

The 'Mean' singer kicked off her four-night run at the Staples Center in Los Angeles, as part of her Speak Now World tour, on Tuesday night (August 23) by inviting the pop prince to join her on stage.

"Who wants to see my friend Justin?" the reigning ACM Entertainer of the Year asked the roaring crowd, before the 17-year-old made a surprise entrance from the back of the stage. The young artists performed his multi-platinum selling hit, 'Baby,' which the songstress also included in her set list in Montreal and Toronto last month.

Later, Taylor took to her Twitter page to express her gratitude for the experience. "You think you've heard LOUD screaming in your life," she tweeted. "Then @justinbieber comes out and does a surprise song during your show. Woah. Unreal."

The teenager wasted no time in returning the love, posting "@taylorswift13 we should do that more often. :) great show! #muchlove" in response.

Taylor welcomed another guest to her concert stage last night (August 24). Singer-songwriter Jason Mraz joined the 21-year-old to perform his Top 10 hit, 'I'm Yours,' resulting in another round of thunderous applause from the concert attendees.

Taylor continues her tour with two more shows at the Staples Center this weekend (August 27 and 28), followed by shows in northern Calif., Oregon and Washington. See her Speak Now World tour schedule here.