"I take these charges very seriously and look forward to clearing my good name," the Atlanta Falcons quarterback said in a prepared statement read outside court by his attorney, Billy Martin.
"I respectfully ask all of you to hold your judgment until all of the facts are shown. Above all, I would like to say to my mom I'm sorry for what she has had to go through in this most trying of times. It has caused pain to my family and I apologize to my family."
Vick and three others entered their pleas in U.S. District Court to charges involving competitive dog fighting, procuring and training pit bulls for fighting, and conducting the enterprise across state lines.
According to the 15-page affidavit, Vick, a cousin and two friends ran a dog fighting business — Bad Newz Kennels — out of Vick's house in Virginia, reports CBS News correspondent Byron Pitts.
In grizzly detail, prosecutors allege dogs that lost in the fighting pit were often shot, hanged, drowned, or electrocuted, adds Pitts.
U.S. Magistrate Dennis W. Dohnal, in releasing the defendants without bond, said the judicial system is grounded on the principle that a defendant is innocent until proven guilty "no matter how heinous the allegations may be."
Among the conditions set for all the defendants is that they surrender their passports, that they not travel outside their immediate area without court approval and that they do not sell or possess any canine.
In addition, Vick was ordered to surrender any animal breeder or kennel license.
Vick and his co-defendants entered their pleas before U.S. District Judge Henry E. Hudson.
"He asserted in a loud and clear voice that he is not guilty of these allegations," Martin said of Vick. "This is going to be a hard-fought trial."
Vick arrived at the courthouse at 3 p.m. in a black sport utility vehicle and was booed by a crowd of hundreds as he emerged.
The allegations detailed in a graphic, 18-page indictment sparked protests by animal rights groups at the headquarters of the NFL and the Falcons. NFL Commissioner Roger Goodell has barred Vick from training camp while the league investigates.
Falcons owner Arthur Blank said the team wanted to suspend Vick for four games, the maximum penalty a team can assess a player, but the NFL asked him to wait. Instead, Blank has told his embattled player to focus on his legal problems, not football.
Thursday, the Falcons opened their first camp under coach Bobby Petrino.
The case began April 25 when investigators conducting a drug search at the home found 66 dogs, including 55 pit bulls, and equipment typically used in dog fighting. They included a "rape stand" that holds aggressive dogs in place for mating and a "breakstick" used to pry open a dog's mouth.
Vick contended he knew nothing about a dog fighting operation at the home, where one of his cousins lived, and said he rarely visited. He has since declined comment, citing his attorney's advice.
Animal rights organizations have seized on the case as an opportunity to raise awareness of the largely underground and always gruesome world of dog fighting, where two dogs are trained to fight to the death, sometimes for hours.
Early Thursday, activists, supporters of the athlete and the media gathered outside the federal courthouse. Some members of People for the Ethical Treatment of Animals dressed in dog costumes and carried signs, including one with the image of a battered pit bull and the words "Dog Fighting Victim." Some people brought their dogs.
According to the indictment filed July 17, dogs not killed in the fighting pit were often shot, hanged, drowned or, in one case, slammed to the ground. The document alleges that Vick was consulted before one losing dog was wet down and electrocuted.
It alleges that the dog fighting operation began in 2001, not long after Vick parlayed a dazzling two-year run as the quarterback at Virginia Tech into being the first overall selection in the NFL Draft. His first contract was for $62 million.
In 2004, he signed a 10-year, $130 million deal, then the richest in league history.
The indictment alleges the fights offered purses as high as $26,000, and that Vick once paid $23,000 to the owner of two pit bulls that had beaten Bad Newz Kennels dogs.
That owner is one of four cooperating witnesses cited in the document.
Charged along with Vick are Purnell A. Peace, 35, of Virginia Beach, Va.; Quanis L. Phillips, 28, of Atlanta; and Tony Taylor, 34, of Hampton, Va. All face up to five years in prison, $250,000 in fines and restitution if convicted of both charges.