Doctors initially had said Camacho was in critical, but stable condition and expected to survive after he was shot Tuesday night in his hometown of Bayamon. But his condition worsened overnight and his heart stopped at one point, said Dr. Ernesto Torres, director of the Centro Medico trauma center in San Juan.
"He's battling minute to minute. This is the most important fight of his life," Torres told The Associated Press outside the hospital in the Puerto Rican capital.
Torres said doctors were trying to determine the boxer's level of brain activity.
The specialists will then consult with other doctors and Camacho's mother, who flew in Wednesday from New York, to discuss whether he should be removed from life support, said Ismael Leandry, a longtime friend and former manager who was also at the hospital.
"We just have to wait to see if 'Macho' gets better. It's a hard battle," Leandry told AP.
Torres said Camacho's mother, Maria, spent about 20 minutes with her son, one of the most dynamic boxing personalities of his era, and was expected to return for a second visit on Wednesday night.
"His mother came and she is devastated," he said. "She knows the prognosis is not at all favorable."
A godson, Widniel Adorno, said the family has discussed the possibility of organ donation but no final decision has been made.
The 50-year-old Camacho was outside a bar in a parked Ford Mustang with a friend when he was shot in the face. The friend, identified as 49-year-old Adrian Mojica Moreno, was killed. Police said two assailants fled in an SUV but no arrests have been made and no motive has been disclosed.
Camacho was rushed to Centro Medico, where doctors initially said the bullet passed through his jaw and lodged in his shoulder. Torres said the bullet damaged three of the four main arteries in his neck and fractured two vertebrae, which could leave him paralyzed if he were to survive.
Steve Tannenbaum, who has also represented Camacho in the past, had been told earlier by friends at the hospital that the boxer would make it.
"This guy is a cat with nine lives. He's been through so much," he said. "If anybody can pull through it will be him."
Friends and family members waited anxiously at the hospital, fondly recalling Camacho's high-energy personality and his powerful skills in the ring.
"He was like a little brother who was always getting into trouble," said former featherweight champion Juan Laporte, a fellow Puerto Rican who grew up and trained with Camacho in New York.
Camacho has been considered one of the more controversial figures in boxing, but also popular among fans and those who worked in the sport.
"The Macho Man was a promoter's dream," renowned promoter Don King told AP. "He excited boxing fans around the world with his inimitable style. He was a nice, amiable guy away from the ring."
King had promoted Camacho but was caught off guard by news of the attack on the former champion. "What a tragedy this is," he said. "I'm very sorry for Hector and his family. My prayers go out to him."
The fighter's last title bout came against then-welterweight champion Oscar De La Hoya in 1997, a loss by unanimous decision. He last fought in May 2010, losing to Saul Duran. Tannenbaum said they were looking at a possible bout in 2013.
"We were talking comeback even though he is 50," he said. "I felt he was capable of it."
Camacho was born in Bayamon, one of the cities that make up the San Juan metropolitan area.
He left Puerto Rico as a child and grew up mostly in New York's Harlem neighborhood, one of the reasons he later earned the nickname "the Harlem Heckler."
He went on to win super lightweight, lightweight and junior welterweight world titles in the 1980s.
Camacho has fought other high-profile bouts in his career against Felix Trinidad, Julio Cesar Chavez and Sugar Ray Leonard. Camacho knocked out Leonard in 1997, ending what was that former champ's final comeback attempt.
Camacho has a career record of 79-6-3.
In recent years, he has divided his time between Puerto Rico and Florida, appearing regularly on Spanish-language television as well as on a reality show called "Es Macho Time!" on YouTube. In San Juan, he had been living in the beach community of Isla Verde, where he would obligingly pose for photos with tourists who recognized him on the street, said former pro boxer Victor "Luvi" Callejas, a neighbor and friend.
"We all know what Macho Camacho has done, but in the last couple of months he hasn't been in any trouble," Callejas said as he kept vigil outside the hospital. "He has been taking it easy. He's been upbeat."
Drug, alcohol and other problems have trailed Camacho since the prime of his boxing career. He was sentenced in 2007 to seven years in prison for the burglary of a computer store in Mississippi. While arresting him on the burglary charge in January 2005, police also found the drug ecstasy.
A judge eventually suspended all but one year of the sentence and gave Camacho probation. He wound up serving two weeks in jail, though, after violating that probation.
His wife also filed domestic abuse complaints against him twice before their divorce several years ago.
Associated Press writers Ben Fox in San Juan, Puerto Rico and David Skretta in Kansas City contributed to this report.