By Ben Fractenberg and Olivia Scheck
MIDTOWN WEST — Rockefeller Center was filled with thousands of little monsters Friday morning.
Around 20,000 screaming fans — according to "Today Show" estimates — packed into the plaza to watch Lady Gaga perform a free concert for the morning news show.
Gaga appeared on stage engulfed in smoke a little after 8:30 p.m. The eccentric singing sensation was dressed in all white as she led off by singing "Bad Romance” to the roaring approval of the morning show crowd.
"It’s like utter mayhem," said Vanessa Denis, 21, who came in from New Jersey for the show.
Some fans seemed overwhelmed by the performance.
"I got goose bumps," said Alana Madden, 21, who lives in Gramercy Park. "It was like a heat wave."
Two fans got worse than that and had to be taken to Bellevue Hospital Center with minor cases of heat exhaustion, the FDNY said.
"All of a sudden, everybody was looking down and she was lying there...the ambulance came over and they took her out," said Denis, who saw a woman faint. After seeing that, Denis decided to step out of the crowd. "I just had to get out."
Emergency officials were called to the scene at 6:58 a.m., two minutes before the "Today Show" went on the air.
The show went on as planned despite the pair of overheated fans.
The Manhattan native belted out "You and I" while playing a white piano for her second song. By the end of the song, she was sprawled out on her bare stomach on the piano singing into the microphone.
The "Today Show" crowd appeared to drink up every moment as Lady Gaga changed into an all black outfit before singing her most recent hit, "Alejandro."
"Hands up, little monsters," she said after walking up to the audience. “I love you, New York!”
Rain started to pour down at the end of the set — almost as if part of the show.
"Let it rain," Gaga yelled while rolling around on the stage.
Then she took a bow with her dancers and left the stage.
Michaela Williamson, 20, traveled with friends all the way from West Virginia on Tuesday for the free concert. They got to 48 Street between Fifth and Sixth Avenues around noon Thursday to get a good spot in line. Fans filled the street armed with snacks and blankets for sleeping on the hard concrete.
"We’re staying with my uncle in The Village, and he surprised us with tickets to Monster Ball," said Williamson who wore a blonde wig and silver studded bikini top.
Williamson and other waiting in line last night were treated to Ray’s pizza, courtesy of Gaga.
"It was the best," said 18-year-old Jose Rivera Jr., who came down from Washingtonville, N.Y. for the show. “A truck came out with like one hundred pizzas.”
The star tweeted Thursday that she would deliver slices for her fans.
"We all screamed and got really excited," said Phillip Shaw, 21, on reading her tweet. Shaw traveled from Connecticut for the show and has been on line since noon Wednesday.
Lady Gaga is scheduled to finish a three-show stand at the famed Madison Square Garden Friday night.