The Rev. Al Sharpton heads to central Florida on Thursday to join the growing crowd protesting the killing of unarmed teenager Trayvon Martin, who was shot by a neighborhood watch leader last month in Sanford.

Sharpton, the outspoken activist and television personality, said he will hold a rally at 7 p.m. at Fort Mellon Park in Sanford, the city in the crosshairs of this national controversy. The rally was moved to the park from a church to accommodate the larger-than-expected turnout.

Sharpton announced on Twitter that his mother passed away early Thursday.  Ada Sharpton, 87, died following a long battle with Alzheimer's and dementia.

Al Sharpton learned of the death as he was boarding a plane to Florida, but said he will still attend the rally for Martin.

"My MOM would have wanted me to," Sharpton tweeted.

IMAGES: Hundreds march in NYC for Fla. teen shot dead

Martin was fatally shot Feb. 26 while walking to the house of his father's fiancee in Sanford after a trip to a convenience store. George Zimmerman, a neighborhood watch leader, said he shot the teen in self-defense.

Zimmerman has not been arrested. A police report describes him as a white male; his family says he is Hispanic.

The shooting and police response is fueling outrage that has reached well beyond Sanford, a racially mixed community 16 miles northeast of Orlando.

Demonstrators of all races crowded into New York's Union Square on Wednesday night, in a a protest that Martin's parents attended. Many of the demonstrators wore hoodies and carried Skittles, the candy Martin had bought on the night he was killed.

More than 900,000 people have signed a petition on Change.org demanding the arrest of the shooter, George Zimmerman.

"George Zimmerman took Trayvon's life for nothing," the teenager's father, Tracy Martin, told the demonstrators. "Our son did not deserve to die. There's nothing that we can say that will bring him back, but I'm here today to assure that justice is served and that no other parents have to go through this again."

"Our son is your son," Martin's mother, Sybrina Fulton, told the crowd. "This is not about a black-and-white thing. This is about a right-and-wrong thing. Justice for Trayvon!"

Earlier, she described her situation as "a nightmare" to Anderson Cooper on his talk show. "It's hard to sleep," she said about her son. "Everything reminds me of him, and the only thing that's fueling us to keep pressing on for justice is the fact that we know that justice will be served."

On Wednesday night, the Sanford City Commissioners passed by 3-2 a no-confidence vote against Sanford Police Chief Bill Lee. It was not immediately clear what impact, if any, that would have.

The vote came a day after Ben Jealous, head of the NAACP, demanded Lee's resignation, accusing him of having mishandled the case by not arresting Zimmerman.

The U.S. Justice Department has launched a civil rights investigation into the shooting.

The incident occurred when Zimmerman, who was patrolling the neighborhood, saw the teen walking home after buying candy and a drink at a convenience store.

Zimmerman called 911 and reported what he described as a suspicious person. Moments later, several neighbors called the emergency number to report a commotion outside.

The 911 tapes, released by police, show that, while some neighbors were on the phone with emergency dispatchers, cries for help followed by a gunshot sounded in the background.