The jury for O.J. Simpson's trial began hearing the details of the case in a Las Vegas court Monday. The former NFL star faces 12 charges, including armed robbery, assault and burglary. (Ethan Miller/Associated Press)

The prosecution in the O.J. Simpson trial began outlining its case to jurors Monday, saying they will learn "the true face" of the former football star charged with robbing two sports memorabilia dealers in a Las Vegas hotel in 2007.

The 61-year-old Simpson and co-accused Clarence (C.J.) Stewart, 54, have pleaded not guilty to 12 charges, including kidnapping, armed robbery and conspiracy.

Prosecutor Chris Owens played part of an audio recording of Simpson, Stewart and four other men when they confronted two salesmen in a Vegas hotel room.

"The audio will show threats, it will show force, it will show demands and it will show the taking of property from the victims in this case," Owens said.

"In our presentation of the evidence, we are going to spend the next few days finding which may be the true face of … Simpson, not necessarily the one he tries to put out to the world," Owens said.

Simpson's lawyer, Yale Galanter, said his client was simply retrieving things that belonged to him but had been stolen years earlier.

"This is not a case about sports memorabilia. It's about personal property," Galanter said Friday.

The prosecution is expected to call about 25 witnesses, including the four other men who were with Simpson and Stewart when they went to the hotel room.

The trial was interrupted Monday afternoon when the prosecution's first witness, Bruce Fromong, pointed to his chest and complained he didn't feel well.

Fromong, 54, an alleged victim in the case, said he has had four previous heart attacks.

The incident occurred several hours into his testimony.

After the judge called for a break, Fromong sat in a chair in a courthouse hallway with a cold compress on his neck, being attended by his wife and court officers.

Both Simpson and Stewart face life in prison, with the possibility of parole, if they are convicted of kidnapping and mandatory prison time if convicted of armed robbery.

With files from the Associated Press