A night of pre-holiday revelry turned to horror this morning when 21 people died in a mangle of bodies as an overflow crowd struggled to escape a nightclub after pepper spray, or Mace, apparently was used to break up a fight.

As many as 1,500 people were at the upstairs E2 nightclub, and so many required medical attention that some lifeless bodies were carried out to ambulances by partygoers as emergency personnel assisted others who had piled up at the club's one double door. Firefighters found four people lying inside a locked kitchen door in cardiac arrest. Others at the front door apparently suffocated or were crushed.

Witnesses described how fun on a Sunday before a national holiday quickly ended in terror, injury and death. In addition to the dead, dozens of others were cut, bruised and suffering from breathing problems that required medical attention. At least 55 were injured, and some were hospitalized.

"They started spraying Mace," Trudell Ferguson, 22, a public school janitor, told a local television station soon after escaping the club. "Everybody was choking and vomiting everywhere. People were getting more anxious and panicky, and they started pushing harder and harder. They got so jammed, they got stuck."

An investigation, with potential criminal penalties, has begun, and authorities said today that several violations of the city's fire code -- including locked or blocked exits -- were immediately evident. In addition, the club was operating in violation of a court order issued in July for building code violations, Fire Commissioner James Joyce said.

After receiving a call of a pregnant woman in distress after 2 a.m., firefighters arrived at the South Michigan Avenue club, which is above the Epitome restaurant, to find a crush of people jammed into a single stairwell, many of them unconsciousness and some gasping for air. People were falling out the front door onto the chilly sidewalk as they broke free.

"People were being trapped underneath you . . . so we're actually standing on people's heads, and we didn't even know it," the Associated Press quoted Amishoov Blackwell, 30, as saying. "It was just bodies laying everywhere."

Blackwell said one man crushed between two people told him, " 'I can't breathe! I want you to hold my hand, man. If I don't make it, tell my mom that I love her!' He just basically collapsed."

"You could see a mound of people," Cory Thomas, 33, who went to the club to pick up two friends, told reporters. "People were stacking on top of each other, screaming and gagging, I guess from the pepper spray. The door got blocked because there were too many people stacked up against it."

It was reminiscent of several other incidents in which concert- or partygoers were trampled. In 1979, 11 people were killed in a crush to get into a concert by the Who in Cincinnati. Eight suffocated in pileup of people trying to get into City College of New York gymnasium for charity basketball game played by rappers in December 1991. And there have been scores of incidents in which people have been trampled to death at soccer matches and religious festivals.

The first responders -- aware that they were overwhelmed -- immediately called for backup, their rescue effort slowed by doors that were locked or obstructed by supplies.

"There were people trying to get out that could not get out," Fire Commissioner Joyce said at a news conference today. "Locked and blocked doors are a contributing factor. We can't explain how management or ownership would allow that."

Joyce said inspectors last visited the restaurant, which has a capacity of 327, in October but did not check the club area because it was not supposed to be in operation. The restaurant and nightclub both advertised heavily, however, and the club was open exceptionally late at night. Joyce said that Lesly Motors Inc. owned the building and that LeMirage held the liquor license.

Worried parents and friends, after learning of the incident, rushed down to look for their loved ones, some going from one hospital to another. Many were still searching hours later. Daryl Bell was out this morning at the nightclub and restaurant looking for his 25-year-old daughter, Daria, who had been at the club.

She calls him every morning at 9, but not today.

"I know she was here last night," he said. "I've tried calling her friends, everyone."

Some people went to the city morgue to identify bodies, most waving off reporters who tried to speak with them. One woman, however, said that her loved one had died in the incident and had bruises all over his face. A stream of pastors, organized in part by Jesse L. Jackson, met with families of the dead.

"There's just an awesome sense of tragedy," Jackson told reporters. "Our worst fears have been realized. The focus must be on prayer. There will be lots of mourning and wailing in the next few days. Children are dead. People cannot contact next of kin."

The club is located on the near South Side, a commercial district two blocks from the McCormick Place convention center and across the street from the office of the Chicago Defender newspaper. Politicians and entertainers such as R. Kelly and P. Diddy have held parties there. And Paul Jakes, an outspoken pastor who is running for mayor, said he and his wife had dined at the first-floor restaurant recently.

Officials said today that the club has a history of code violations, most of them related to inadequate exits. It was the site, though called the Clique at the time, where Mike Tyson was accused in 1996 of attacking a 25-year-old beautician from Gary, Ind. Tyson was not charged. Now called E2, the club and its immediate vicinity have been involved in 80 incidents in the past two years in which police have responded to public disturbances, said Police Chief Terry Hillard.

Officials said they believe the trampling incident began with an altercation between two or more people that became more dangerous when security at the club sprayed an irritant into the air to break up the fight. But Hillard said that, at this point, nothing is known for sure. Police do not know exactly how many people were in the club or who was initially involved in the fight, he said. And they are unsure whether pepper spray was used or by whom.

"There is conflicting information," said Hillard, noting that detectives wanted anyone who was in the club Saturday night to contact them. "We are interviewing security personnel and seeking witnesses to determine what happened."

Chicago Mayor Richard M. Daley, whose mother died Sunday, did not make a public appearance today. But he vowed in a statement to "take whatever steps necessary to make sure nothing like this ever happens again."

A television image shows some of the guests trying to exit Chicago's E2 nightclub. Dorothy Myers, front left, appears with family members at a news conference Monday about her son Antonio's death at the E2 nightclub.