By the CNN Wire Staff
updated 9:55 PM EDT, Fri May 11, 2012

(CNN) -- Saying he had no discretion under state law, a judge sentenced a Jacksonville, Florida, woman to 20 years in prison Friday for firing a warning shot in an effort to scare off her abusive husband.

Marissa Alexander unsuccessfully tried to use Florida's controversial "stand your ground" law to derail the prosecution, but a jury in March convicted her of aggravated assault after just 12 minutes of deliberation.

The case, which was prosecuted by the same state attorney who is handling the Trayvon Martin case, has gained the attention of civil rights leaders who say the African-American woman was persecuted because of her race.

After the sentencing, Rep. Corrine Brown confronted State Attorney Angela Corey in the hallway, accusing her of being overzealous, according to video from CNN affiliate WJXT.

"There is no justification for 20 years," Brown told Corey during an exchange frequently interrupted by onlookers. "All the community was asking for was mercy and justice," she said.

Corey said she had offered Alexander a plea bargain that would have resulted in a three-year prison sentence, but Alexander chose to take the case to a jury trial, where a conviction would carry a mandatory sentence under a Florida law known as "10-20-life."

The law mandates increased penalties for some felonies, including aggravated assault, in which a gun is carried or used.

Corey said the case deserved to be prosecuted because Alexander fired in the direction of a room where two children were standing.

Alexander said she was attempting to flee her husband, Rico Gray, on August 1, 2010, when she picked up a handgun and fired a shot into a wall.

She said her husband had read cell phone text messages that she had written to her ex-husband, got angry and tried to strangle her.

She said she escaped and ran to the garage, intending to drive away. But, she said, she forgot her keys, so she picked up her gun and went back into the house. She said her husband threatened to kill her, so she fired one shot.

"I believe when he threatened to kill me, that's what he was absolutely going to do," she said. "That's what he intended to do. Had I not discharged my weapon at that point, I would not be here."

Alexander's attorneys tried to use the state law that allows people to use potentially deadly force anywhere they feel reasonably threatened with serious harm or death.

But a previous judge in the case rejected the request, saying Alexander's decision to go back into the house was not consistent with someone in fear for her safety, according to the Florida Times Union newspaper.

A jury convicted Alexander in March and Judge James Daniel denied her request for a new trial in April.

Daniel handed down the sentence Friday after an emotional sentencing hearing during which Alexander's parents, 11-year-old daughter and pastor spoke on her behalf.

Several people had to be escorted from the courtroom after breaking out singing and chanting about a perceived lack of justice in the case, but Daniel made a point to say that he had no choice under state law.

"Under the state's 10-20-life law, a conviction for aggravated assault where a firearm has been discharged carries a minimum and maximum sentence of 20 years without regarding to any extenuating or mitigating circumstances that may be present, such as those in this case," Daniel said.

Brown, the Jacksonville congresswoman, told reporters after the sentencing that the case was a product of "institutional racism."

"She was overcharged by the prosecutor. Period," Brown said. "She never should have been charged."

Brown has been more complimentary about Corey's work in the Trayvon Martin case, where her office filed second degree murder charges against neighborhood watch volunteer George Zimmerman in the February 26 death of the unarmed African-American teen-ager.

That case provoked nationwide protests demanding Zimmerman's arrest after an initial police investigation released him under the "stand your ground" law.