Usher’s Broadway Debut Comes Off Without A Hitch — Except For That Cell Phone …

Singer finds some unexpected competition from Cassie.

NEW YORK — Tuesday night on Broadway, the atmosphere at the Ambassador Theater felt like the opening date of a concert tour. Usher was making his grand debut on Broadway as the immaculately dressed Billy Flynn in “Chicago” and everything was going according to plan. He was onstage delivering a song, when all of sudden, Usher found himself battling for the audience’s attention — pitted against a ringtone of Cassie’s “Me & U.”

An audience member had broken the theater’s rules and left his or her cell phone on. The ringer was as loud as the music coming from the stage — and Usher was convinced it was coming from the phone of his former mentor, J Records head Clive Davis.

“I gotta get in Clive Davis’ a–,” Usher said after the show, on the red carpet of his afterparty at Manhattan’s Nikki Beach. “His phone was ringing right in the middle of the play. Can you believe that?”

Davis, the legendary music mogul, would later say with a smile that Usher was surely kidding — and he doesn’t even have a phone. “If I had a phone, how could he tell it was me?” he asked.

That minor hiccup notwithstanding, Usher’s turn in Chicago was essentially flawless, which should come as little surprise. He’s obviously spent most of his life focusing on singing and dancing — his top hat in one of the closing numbers will give some flashbacks of “My Way.” He added a little bit of soul to songs such as “All I Care About,” and even made “Razzle Dazzle” seem cool. Although his dance routines on Broadway aren’t as intricate or fast-paced as the ones for his own classics like “Caught Up” and “Yeah!,” he managed to add a bit of his own hip-hop flavor to the play: An extra spin here, a little more swagger on a foot-glide there.

As an actor, Usher had no problem getting his smooth on, transforming himself into the manipulative but charming “silver-tongued prince of the courtroom,” Flynn. He’s a lawyer trying to get a couple of homicidal hotties off of death row — for the small price of $5,000 each.

“I got a great response my first night,” Usher said after the show. “I’m happy I could share this moment with so many great people. Of course, you have opening-night jitters on anything you do, especially Broadway.” (See ” ‘Broadway Is No Joke’: An Inside Look At Usher’s ‘Chicago’ Rehearsals” and “Usher Goes To Broadway, Finds Kindred Spirit In Richard Gere.” )

Along with Clive Davis, Penelope Cruz, Ben Vereen, Mario, Gabrielle Union, Swizz Beatz, Rosie Perez, Jill Marie Jones, Bryan Barber, Phylicia Rashad and Oprah Winfrey’s best friend Gayle King were in the audience to cheer the man who sits atop the R&B food chain.

“I was very impressed,” “Idlewild” director Barber said of Usher’s performance. “He was very convincing. I love what Usher brings with his vocals.”

“It’s something I will remember 10 years from now,” Mario said of seeing his friend on a Broadway stage, calling the performance “effortless.” “I’m enjoying that he respects the art of it.”

Broadway veteran Bianca Marroquin, who plays Roxie Hart (one of the aforementioned homicidal hotties) in “Chicago” and spends a considerable amount of time with Usher, said the Grammy winner is “wonderful.”

“We’re so impressed, so proud, we can’t wait to see what else he’s gonna do,” she enthused.

A few hundred people mingled outside the theater after the show let out, hoping to get a glimpse of Usher as he made his exit in a pearly white 1936 Auburn Speedster (promotional posters for the play find him posing in front of one). However, his dramatic exit was delayed as the crowd mobbed his vehicle, leaving no room to drive off.

The afterparty had all the trappings of a Hollywood premiere, with two white Rolls-Royce Phantoms lining the streets (Swizz Beatz pulled up in one) along with SUVs, Benzes and at least one Maybach. Inside the hot club, members of the “Chicago” cast were seen dancing to Justin Timberlake’s “SexyBack,” actor Keith David was taking pictures with friends, and several ladies formed their own circle and got down to Nelly Furtado’s “Promiscuous.” Ben Vereen Jr. joined in the excitement, jumping on a chair and dancing when the DJ threw on Beyoncé’s “Deja Vu.”

Even stars who missed the play, like Busta Rhymes, Amerie and Consequence, came out to congratulate the man of the evening.