Trayvon Martin's mother takes witness stand

MIKE SCHNEIDER and KYLE HIGHTOWER
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Defense attorneys Mark O'Mara, left, Don West, center, stand with George Zimmerman during Zimmerman's trial in Seminole circuit court, in Sanford, Fla., Wednesday, July 3, 2013. Zimmerman is charged with second-degree murder in the 2012 fatal shooting of slain teen Trayvon Martin. (AP Photo/Orlando Sentinel, Jacob Langston, Pool)

SANFORD, Fla. (AP) — The mother of 17-year-old Trayvon Martin is on the witness stand at the trial of the neighborhood watch volunteer who is charged with murder in the fatal shooting of her son.

Sybrina Fulton was called by prosecutors to testify Friday. It's likely she will be asked about 911 calls that capture a struggle between Martin and George Zimmerman.

Zimmerman is pleading not guilty to second-degree murder, claiming he shot Martin in self-defense.

Fulton posted on Twitter Friday, saying "I pray that God give me the strength to properly represent my angel Trayvon."

Martin's parents say the screams are those of their son, while Zimmerman's family claims Zimmerman is yelling on the calls.

Attorneys say identifying the screams can show who the aggressor was in the fight.