Vick was jeered by a crowd as he went into court. He and three others entered their pleas in U.S. District Court to conspiracy charges involving competitive dogfighting, procuring and training pit bulls for fighting, and conducting the enterprise across state lines. Federal prosecutors say the operation - known as Bad Newz Kennels - was run on Vick's property in Surry County.
"I take these charges very seriously and look forward to clearing my good name," Vick said in a statement read outside court by Billy Martin, his lawyer.
"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."
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 dog. In addition, Vick was ordered to surrender any animal breeder or kennel license.
The co-defendants made 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.
"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. Wearing a dark suit and blue shirt, the quarterback looked straight ahead as he walked up the ramp to the courthouse. He did not respond to reporters.
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 the player to concentrate on his legal problems, not football.
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 dogfighting. 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 dogfighting operation at the home, where one of his cousins lived, and said he rarely visited. He has since declined comment, citing his lawyer's advice.
Attorney Lawrence Woodward, who has also represented Allen Iverson and Vick's younger brother, Marcus, has not returned several phone messages.
Charged along with Vick are Purnell A. Peace, 35, of Virginia Beach; Quanis L. Phillips, 28, of Atlanta; and Tony Taylor, 34, of Hampton. They all face up to six years in prison, $350,000 in fines and restitution if convicted.
Animal rights organizations have seized on the case as an opportunity to raise awareness of the largely underground and always gruesome world of dogfighting, where two dogs are trained to fight to the death - sometimes for hours - until the end.
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 "Dogfighting 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 says Vick was consulted before one losing dog was wet down and electrocuted.
It alleges that the dogfighting 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 says 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.
Associated Press Writers Dionne Walker, Larry O'Dell and Michael Felberbaum contributed to this report.