WICHITA, Kan. -- A pro-life activist pleaded not guilty Tuesday to opening fire on late-term abortion provider George Tiller.

Scott Roeder, 51, is accused of threatening two ushers who tried to stop him during the May 31 shooting in the foyer of Tiller's church.

Roeder, dressed in a jacket and tie but shackled at his ankles, pleaded not guilty to first-degree murder and aggravated assault charges after witnesses described seeing him shoot Tiller and speed away from the church. Roeder's public defender entered the plea on Roeder's behalf and the Kansas City, Mo. man did not speak during the hearing.

If convicted, Roeder is likely to face life in prison. Prosecutors have said they will not pursue the death penalty, and Roeder would be eligible for parole after 25 years.

Tiller had been the target of protests at his Wichita clinic, where he practiced as one of the nation's few providers of late-term abortions. He was shot in both arms by a pro-life activist in 1993.

For weeks, Roeder has refused to discuss his alleged role in Tiller's death, advocating in general terms for justifiable homicide -- which he has repeatedly said is an acceptable action to protect "unborn children."

In rambling phone and jailhouse interviews since his arrest, Roeder has said he would be pleased if others took action to stop abortion by any means necessary.

"Violence is not wrong in all situations, so if it takes that -- then if it is done righteously -- then, if it's done, it is OK," Roeder has said.

Unlike his peers, Tiller embraced a high profile even after being wounded in 1993. His clinic, heavily fortified after a bombing in 1986, became the target of both peaceful and violent protests. In 1991, a 45-day "Summer of Mercy" campaign organized by Operation Rescue drew thousands of pro-life demonstrators to Wichita. More than 2,700 arrests resulted.

During Tuesday's hearing, an usher at the church in Wichita, Gary Hoepner, testified that he and Tiller were chatting when a man walked through the door, put a gun to Tiller's head and shot him. Hoepner identified the man as Roeder.

Hoepner said he wasn't sure if the weapon used to kill Tiller was real until he saw Tiller fall to the ground. He said he followed the shooter, whom he identified as Roeder, out of the church but stopped after Roeder warned him.

"`I've got a gun and I'll shoot you,"' Hoepner recalled the gunman saying. "I believed him and I stopped."

He said he later called police to give them the license plate number on the shooter's car.

But defense attorney Steve Osburn said some of Hoepner's testimony was based on assumptions, including whether the gunman spoke directly to the other usher. Osburn also asked Hoepner if he told police he heard the gunman say something along the line of "Lord, Forgive me." Hoepner said he did.

Later Tuesday, a judge ordered Roeder held on $20 million bond and set a trial date for Sept. 21.

After Tiller's death, his family said that they would permanently shut the doors to the clinic. The facility's signage has been taken down, and a tall privacy fence of solid boards surrounds the building.

Mainstream abortion opponents have been swift to condemn Tiller's death. Two of the movement's leading groups, Operation Rescue and Kansans for Life, have disputed that Roeder belonged or donated money to either group.