'American Idol' Series Finale Recap: Alumni Cameos and Star-Studded Performances

— -- Thursday’s series finale of "American Idol" paid tribute to the past 15 seasons, with surprise appearances and star-studded performances from past finalists -- including Kelly Clarkson, Carrie Underwood and Jennifer Hudson -- before the final American Idol" was crowned.

Host Ryan Seacrest revealed that Trent Harmon is the winner of the Fox show’s 15th and final season. La’Porsha Renae came in second.

Though Harmon took home the big prize -- a contract with Big Machine Records -- Renae didn’t walk away empty-handed. The singer will also be joining the Universal Music Group family, signing with 19/Big Machine/Motown Records, in partnership with Big Machine. Both finalists also received a Ford Fusion.

Here are Thursday’s highlights:

The show opened with a performance from this season’s top 10, who sang Barry Manilow’s “One Voice.” Over the course of the song, they were gradually joined by past "Idol" winners and contestants, including Kellie Pickler, Taylor Hicks, Katharine McPhee, Constantine Maroulis and others.

Seacrest took the stage and introduced Brian Dunkleman, who served as his co-host for the show’s first season but didn’t stick around for subsequent seasons. Dunkleman joked that he knew the “pain of life without Idol,” but assured Seacrest that being unemployed “is not that bad.” When Seacrest noted that he had “a radio gig and some other stuff going on,” Dunkleman replied, “That’s what I said too.”

La’Porsha and Trent then teamed up for “It Takes Two,” first performed by season-one finalists Kelly Clarkson and Justin Guarini.

The reunion continued with Jordin Sparks and Tamyra Gray joining season-eight and -nine judge Kara DioGuardi for Pink’s “Sober.” Season 11’s Colton Dixon sang his song “Through All of It,” and Sparks returned for her hit song “No Air” with Guarini. Season two’s Kimberley Locke sang “8th World Wonder,” leading into Guarini, Sparks and Gray singing Little Big Town's “Girl Crush.” Season eight’s Allison Iraheta sang Meghan Trainor’s “No,” seguing into a mashup of Larry Pratt’s “Pants on the Ground” audition. Season 10’s Pia Toscano closed the set with “All By Myself” by Celine Dion.

The pregnant Clarkson couldn’t attend in person, but appeared in a pre-taped segment performing a medley of her hits that included “Miss Independent,” “Breakaway,” “Since U Been Gone,” “Because of You,” “My Life Would Suck Without You,” “Don’t You Wanna Stay,” “Stronger (What Doesn’t Kill You),” “Heartbeat Song” and her winning single, “A Moment Like This.”

Several of the show's rockers, including Chris Daughtry, Bo Bice, Maroulis and season-13 champ Caleb Johnson, joined forces for a rock & roll medley.

Judge Keith Urban got into the act, teaming up with Carrie Underwood for the Stevie Nicks and Tom Petty duet, “Stop Draggin’ My Heart Around.”

Next, "Idol" country artists, among them Diana DeGarmo, Bucky Covington, Lauren Alaina, Kellie Pickler and Scotty McCreery, took center stage for a medley of country hits.

Judge Harry Connick, Jr. brought Ellis Marsalis Center for Music student and fellow New Orleans native Marley Fletcher onstage to perform a duet of Louis Armstrong's "What a Wonderful World." Fletcher is one of the beneficiaries of "American Idol" charities, which have raised over $185 million for children in underserved communities.

Next up, Katharine McPhee and Casey James joined forces for Lady Antebellum’s “Need You Now,” and season seven’s Carly Smithson tackled Dolly Parton’s “Here You Come Again.” Season-two champ Ruben Studdard and Amber Holcomb sang The Beatles’ "Here There and Everywhere,” and Jessica Sanchez performed Celine Dion’s “The Prayer.”

Judge Jennifer Lopez followed with a medley of songs from her All I Have residency at Caesars Palace in Las Vegas, including “Ain’t Your Mama” and “Let’s Get Loud.”

LaToya London, Fantasia Barrino and Jennifer Hudson -- season three's "Three Divas" -- reunited for an a cappella performance of Simon & Garfunkel's “Bridge Over Troubled Water.” Hudson followed with “Remember the Music,” her song from the Fox drama Empire. Brandon Rogers, Clark Beckham, Danny Gokey, George Huff and Elliott Yamin continued the nostalgic trip with The Temptations' "Ain't Too Proud to Beg." Fantasia delivered a solo performance of her single “Ugly,” and then London came back out to sing Aretha Franklin and George Michael's "I Knew You Were Waiting (for Me)" with Taylor Hicks. Joshua Ledet from season 11 then sang James Brown's “It’s A Man’s Man’s Man’s World.” Season-12 winner Candice Glover and season six’s Melinda Doolittle closed out the medley.

Phillip Phillips, Lee DeWyze, David Cook, Nick Fradiani and Kris Allen performed a tribute to David Bowie that included the late rocker’s songs “Starman,” “Rebel Rebel” and “The Jean Genie.”Original judges Paula Abdul and Randy Jackson then honored Seacrest -- the only member of the original cast to last the entire series -- with the famous Idol sign. They were joined by Simon Cowell, who took the opportunity to deliver an apology on behalf of the panel for all the harsh comments they made to contestants.

Seacrest then got a surprise visit from William Hung, who bounded onto the stage to sing his audition song, “She Bangs,” by Ricky Martin.

Underwood closed out the performances with her 2014 hit, “Something in the Water.”