Gary Hoepner, an usher at the Reformation Lutheran Church in Wichita, testified Monday that he saw 51-year-old Scott Roeder walk into the church May 31, put the gun "point blank" to Tiller's head and pull the trigger.
During an hour of testimony, Hoepner described the chaotic scene as the single shot rang out.
"It was a loud, popping noise," Hoepner testified. "Then George fell, and I repeated 'Oh my God, Oh my God' several times."
Hoepner says he ran after Roeder but stopped after Roeder threatened to shoot him and another usher.
CBS Affiliate KWCH correspondent Cliff Judy, who is live blogging the trial, reports that Hoepner said, "I thought about my wife who wasn't there with me that day . . . He could have shot me, and we'd have two victims. I'd just seen him shoot one person. I had no reason to think he wouldn't shoot me."
As Roeder ran to his car, Hoepner testified that he called for another church member to get his license plate number while he retrieved his cell phone to call 911.
During cross examination by defense attorney Mark Rudy, Hoepner testified that he had seen Roeder at the church before the day of the shooting, and that Roeder said "Lord forgive me," when he threatened Hoepner.
Roeder, 51, has publicly admitted he killed Tiller but has pleaded not guilty to charges of first-degree murder and aggravated assault in the case. The Kansas City, Mo., man said in a court filing that the trial would be a "charade" if he were not allowed to argue that the killing was necessary to save "preborn babies" from abortion.
District Judge Warren Wilbert banned a so-called necessity defense, which would be used to argue Roeder should be acquitted, and insisted the trial would not turn into a battle over abortion.
But the judge galvanized both sides of the debate when he refused to bar the defense from trying for a conviction on the lesser charge of voluntary manslaughter by arguing Roeder believed Tiller's killing would save unborn children.
Wilbert won't rule on whether to let jurors consider the lesser charge until after the defense rests its case.
Monday marked the second day of testimony in the trial.
Prosecutors showed photographs of the crime scene: the entrance to the church's sanctuary, a refreshment table, a slumped body, and pools of blood.
Also beginning his testimony today is Keith Martin, another church member who had seen Roeder at the church the previous Sunday.
Martin said after the shooter fled he ran out of the church to the parking lot and tried to block Roeder's car, when Roeder pointed his gun at Martin, demanding he move out of the way.
KWCH's Judy reports that Martin tried to get a good look at the gun to determine if Roder had any more rounds, but ended up moving, believing he'd be shot.
Security in and around the courthouse has been heavy; a bomb-sniffing dog has been brought in, and spectators must show photo identification and are not allowed to bring coats or purses into the courtroom.
Prosecutors have appeared determined to keep any mention of abortion out of what they portray as a simple case of murder. That led last Friday to what was likely the first of many skirmishes over the use of what attorneys are calling, outside the jury's hearing, the "a-word." The judge warned defense attorney Mark Rudy in cross-examining one witness that he should not use the word abortion unless the witness first used it himself.
Key to the government's murder case will be presenting evidence of premeditation. Prosecutors said Roeder meticulously planned the killing, stalked the doctor at his church in Wichita, and practiced firing his newly purchased gun at his brother's secluded home the day before the killing.
On Friday, the prosecution began presenting a murder case focused on emotional eyewitness testimony, recordings of frantic 911 calls and photos of Tiller's body lying in a pool of blood in his church foyer.
DNA evidence linking Tiller to Roeder, forensic analyses of bullet casings and video of Roeder at local hotels are expected to follow - but no mention of abortion, at least for as long as they can avoid it.
Roeder's public defenders have yet to make their opening statements, but are expected to try to build a case for a conviction of voluntary manslaughter. The judge has warned them that would be difficult because the facts indicate Tiller posed no immediate danger while acting as an usher in church.
In Kansas, voluntary manslaughter is defined as "an unreasonable but honest belief that circumstances existed that justified deadly force." Such a conviction for someone with little criminal history would bring a sentence closer to five years, compared to the life sentence Roeder faces if found guilty of first-degree murder.