A NAACP petition calling for a federal prosecution of George Zimmerman collected 225,000 signatures in the hours after he was acquitted of killing Trayvon Martin, but the effort was temporarily cut short when the site crashed under the heavy traffic.
The rush to add to the petition -- which had more than 350,000 signatures by noon today -- was matched by the outpouring of anger on social media.
Protests, which police had feared could turn violent, were muted, but the rhetoric was impassioned and often framed around the issue of race. Zimmerman, 29, is a white Hispanic and Martin, 17, was black.
New York City Mayor Michael Bloomberg issued a statement that put the onus on gun laws.
"Sadly, all the facts in this tragic case will probably never be known. But one fact has long been crystal clear: 'shoot first' laws like those in Florida can inspire dangerous vigilantism and protect those who act recklessly with guns," Bloomberg said. "Such laws – drafted by gun lobby extremists in Washington – encourage deadly confrontations by enabling people to shoot first and argue 'justifiable homicide' later."
Immediately after Saturday night's acquittal, the NAACP said it was "outraged" by the Florida jury's verdict and called on the Justice Department to prosecute Zimmerman for civil rights violations.
The civil rights organization also posted a petition on their site calling for the prosecution.
"The most fundamental of civil rights -- the right to life – was violated the night George Zimmerman stalked and then took the life of Trayvon Martin," the petition read. "We ask that the Department of Justice file civil rights charges against Mr. Zimmerman for this egregious violation. Please address the travesties of the tragic death of Trayvon Martin by acting today."
Zimmerman shot and killed Martin in Sanford, Fla., on Feb. 26, 2012. Zimmerman, 29, maintains he shot Martin, 17, in self-defense, while the state argued that Zimmerman "profiled" Martin and concluded he was a criminal.
NAACP spokesman Derek Turner told ABCNews.com that the petition garnered approximately 225,000 signatures between the hours of 11 p.m. Saturday and 3 a.m. Sunday.
Sometime overnight, the NAACP's website crashed, Turner said, because of "too many viewers and too many hits."
The NAACP's website was still inaccessible as of this morning, and Turner said the organization is working to get it back up and running. He did not know how long the website had been down, but he was last able to access it around 2:30 a.m. Sunday, he said.
The same petition was also made available on MoveOn.org in partnership with the NAACP early Sunday morning. It gained more than 130,000 supporters by noon today.
"Our members, like so many Americans, are outraged at the verdict. Justice has not been served. The facts are clear: a 17-year-old boy is dead because George Zimmerman shot him. This is a sad day for our country and our justice system," MoveOn.org Civic Action executive director Anna Galland said in a statement.
An investigation had previously been opened by the Justice Department, and the department said Saturday night, "The department continues to evaluate the evidence generated during the federal investigation, as well as the evidence and testimony from the state trial."
ABC News anchor Dan Abrams said it is unlikely the civil rights division will file charges against Zimmerman "because they can't win."
"There will be a federal investigation. They will publicly discuss it. The civil rights division will not file. " Abrams said. "They won't win, and they know that."
Police departments across south Florida had been braced for possible violence in response to the acquittal, but while reaction in social media has been heated the handful of protests in San Francisco, Los Angeles and Atlanta were muted and peaceful.
But in Oakland, Calif., about 100 demonstrators took to the streets, smashing the windows of businesses, starting small fires in the streets, and vandalizing a police car, the Associated Press reported.
Benjamin Crump, a lawyer for the Martin family, acknowledged the disappointment of Trayvon Martin's supporters, but he urged them not to resort to violence.
"For Trayvon to rest in peace, we must all be peaceful," Crump said.
ABC News' Shushannah Walshe contributed to this report