"American Idol" host Ryan Seacrest signed off from Idol for the last time on Thursday night after helming a massive alumni reunion on the reality competition's "farewell" finale.

Kelly Clarkson, Carrie Underwood, Ruben Studdard, Jennifer Hudson, Fantasia Barrino and dozens of other past contestants and winners from the show's 15-year history came together Thursday night to say goodbye to the "Idol" stage.

The farewell episode included a string of performances, with plenty getting special attention on social media including Jessica Sanchez's bombshell rendition of Celine Dion's "The Prayer," and judge Jennifer Lopez's glamorous debut of "Ain't Your Mama."

At the end of the night's performances, Paula Abdul, Randy Jackson and Simon Cowell -- the show's original judges -- made an appearance during the finale to gift Seacrest with a massive American Idol neon sign.

This season's finalists -- La'Porsha Renae and Trent Harmon -- sang "It Takes Two" to kick off the night ahead of Harmon's eventual win.

The song was first performed on Idol by the first season's finalists, Kelly Clarkson and Justin Guarini.

Jordin Sparks, Tamyra Gray and season-eight judge Kara DioGuardi also joined the fun with a version of Pink's "Sober" before cameras turned to Colton Dixon, Kimberley Locke and Allison Iraheta.

Pia Toscano ended the segment with Celine Dion's "All By Myself."

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Original "American Idol" champion Kelly Clarkson was not able to attend the competition's farewell episode, but did appear in a pre-taped segment where she performed several of her biggest hits including "Miss Independent," "Breakaway," and "My Life Would Suck Without You."

Chris Daughter, Bo Bice, Maroulis and Caleb Johnson came together next for a rock medley, leading in to current judge Keith Urban's performance of "Stop Draggin' My Heart Around," with superstar Underwood.

A team of country artists, including Diana Degarmo, Bucky Covington, Lauren Alaina, Kellie Pickler and Scotty McCreery then came together with another medley performance.

"American Idol" judge Harry Connick, Jr. also performed, but not before mentioning the work "Idol" has done for charities like those who help hurricane Katrina victims.

He was accompanied by a young Ellis Marsalis Center for Music student Marley Fletcher to sing Louis Armstrong's "What A Wonderful World."

Katharine McPhee, Casey James and Carly Smithson were up next, singing "Need You Now" and "Here You Come Again," before Season 2 winner Ruben Studdard and Amber Holcomb segued into "Here There And Everywhere," and Jessica Sanchez finished off with a powerful version of "The Prayer."

LaToya London, Fantasia Barrino and Jennifer Hudson were next with an a cappella performance. Brandon Rogers, Clark Beckham, Danny Gokey, George Huff and Elliott Yamin sang a throwback "Ain't Too Proud to Beg" including classic Temptations dance moves.

David Bowie was then honored with a group performance by guitar-slinging "American Idol" winners Phillip Phillips, Lee Dewyze, David Cook, Nick Fradiani and Kris Allen.

The night was closed by a surprise performance by beloved auditioner William Hung, who sang his famous rendition of "She Bangs" by Ricky Martin.

Underwood followed soon after with "Something in the Water."