NEW YORK (WABC) -- On week 3 of the Junior Guzman murder trial, there were audible gasps in the courtroom when the prosecution called their star witness to the stand.
Kevin Alvarez, one of the alleged gang members already indicted on murder charges, flipped on his co-defendants.
Alvarez, indicted on second degree murder charges for the violent murder of 15-year-old Lesandro Guzman-Feliz, known as Junior, already pleaded guilty and is cooperating with the prosecution.
In front of a packed courtroom on Monday, a nervous Alvarez identified the five defendants standing trial as gang members who brutally stabbed Junior last June in the Bronx.
Alvarez says he drove one of the four vehicles used to chase Junior.
He said Junior first tried to run into St. Barnabas Hospital for safety, but he blocked his path, tried to grab him and that's when Junior ran to the bodega a block away.
Alvarez says Junior looked surprised and scared and when gang members surrounded him, he ran.
The gang members thought Junior was involved in a rival gang shooting.
Alvarez then started sobbing on the stand.
"I think he's terrified. I think he's definitely scared and hopefully they all get what they deserve," Jennifer Weisel, a Junior supporter, said.
Alvarez said he was the one who pushed through the door where Junior was hiding, behind the bodega counter, and punched and kicked Junior repeatedly in the face and body.
Junior then fell to the floor and that's when Alvarez said he, along with another alleged gang member, kept kicking him and dragging him outside while Junior yelled over and over, "Stop, I didn't have nothing to do with it!"
Both Alvarez and Junior's mom sobbed in court as the prosecution then played footage from inside the bodega.
Christopher Carrion, a legal analyst who's been observing the proceedings, says Alvarez's emotional testimony will help the prosecution.
"It does help. It makes him look like a human. He's not a robot who was extensively prepared for this. And when he characterized some of what he observed like Junior being in fear, he got scared and ran away, that really brings things home," Carrion said.
By the end of his testimony, Alvarez wouldn't even look in the direction of his co-defendants.
Defendant Jose Muniz became aggressive, yelling in Spanish that he wasn't getting a fair trial.
He had to be restrained by court officers then removed.
Then a second defendant, Jonaiki Estrella, wanted to leave, but his attorney convinced him to stay.
Alvarez will be cross-examined on Tuesday.
* More New York City news
* Send us a news tip
* Download the abc7NY app for breaking news alerts
* Follow us on YouTube
Justice for Junior trial: Key witness for the prosecution takes the stand