At 4 o'clock on Sunday afternoon, millions of Americans, perhaps as many as a third of the nation's people, will gather around television sets in living rooms and taverns for the largest single spectator sports event of the year, Super Bowl XVI.

Not surprisingly, it also is the largest betting event of the year. The estimates of the total amount wagered on the outcome range as high as $10 billion.

"Nobody really knows how much it is," says Bob Martin, the Las Vegas linemaker who sets the point spreads that most bookmakers in the nation follow. "It could be $500 million, it could be $1 billion, it could be $5 billion. The Super Bowl is the biggest, but it's impossible to know just how much is bet on it. Any guess that anybody makes is as good as any other guess."

Just as the Super Bowl has moved in only 16 years to the summit of American spectator sports, so has sports betting increased in a similar time span. It has become a complex and sophisticated industry in which millions of dollars change hands each week.

Gary Austin, for example, a former salesman in Southern California, found that sports gambling offered him the opportunity to quit his job to pursue his avocation--and make money in the process.

Austin is a professional sports bettor. He says he wagers up to $30 million a year. He runs a sports handicapping service in Las Vegas, and more than 1,000 people are willing to pay him $1,200 a season or $450 a month for his suggestions on how to bet.

"I started out betting with friends, $5, maybe $10, on a game," said Austin. "Then I started betting with a bookmaker, $100, $200, $500. About three years ago I won a handicapping contest. People were offering me money for my selections, so I went into the business."

Although it has long been said that betting and spectator sports are inseparable, sports gambling in recent years has reached unprecedented levels. Some estimates place the total yearly amount wagered on sports events, excluding horse racing, at $60 billion.

"It is certainly a multibillion dollar a year industry," says Charles H. Morin, a Washington lawyer who in the mid-1970s was chairman of the now defunct Commission on the Review of the National Policy Toward Gambling.

Sports handicapping services like Austin's have become a major part of the gambling business. Ed Horowitz, a certified public accountant and former college professor, left a career in computers five years ago to start such a business.

As many as 1,500 people from across the continental United States, Mexico, Canada and Hawaii paid $165 a weekend to telephone Horowitz for his picks during the football season, a weekly gross of $114,000.

Last March, Horowitz's company, American Sports Advisors, became the first sports-predicting organization to go public when 3.2 million shares of its stock were offered for sale in over-the-counter trading at 60 cents a share.

"It sold out in a month," said Richard Gray, the president of Silver, Gray & Co. Inc., the brokerage firm underwriting the sale. "I guess the public is gambling conscious. They grabbed it up like there was no tomorrow and the company raised $1.92 million.

"Most of the companies offering these low-priced stocks need the public's money to survive, but this company was standing on its own two feet. The principals are very knowledgeable and very successful. This year will be a record year. Net income after taxes for the first three quarters was $279,625. Last year it was $22,295."

The foremost linemaker is Martin, 63, who ran a bookmaking operation in Washington from the early 1950s to the early 1960s. "I've been a player for 40 years, betting and what not," says Martin.

"There are so many things you consider, but after a while you get a gut feeling for the way people are going to bet. Let's say I think Dallas and the 49ers are even. I know more people are going to bet on Dallas so I'll put Dallas up by 2 1/2 in order to attract money for San Francisco."

The whole purpose of the line, says Martin, is not to predict the outcome of the game but to equalize the sums wagered on either side.

"Betting," says Marv Harshman, basketball coach at the University of Washington, "seems to be a way of life in our society."

Although sports gambling is illegal in every state but Nevada and is widely reputed to be linked to organized crime in many areas of the country, the Justice Department says it puts a low priority on enforcement of antigambling laws. Fewer than one-quarter of those convicted of gambling offenses receive jail sentences, according to the final report of the National Commission on the Review of the National Policy Toward Gambling.

"We're pretty much out of the gambling business," said John Russell, a Justice Department spokesman. Other federal law enforcement officials say criminal activity such as organized crime involvement in labor racketeering, corruption of public officials, infiltration of legitimate business, and illegal drug traffic has taken precedence over gambling enforcement on the Justice Department agenda, although traditionally local authorities conduct a series of gambling raids in the weeks surrounding the end of the professional football season and the playoffs.

The Commission on the Review of the National Policy Toward Gambling was established in 1970 to study and make recommendations concerning gambling trends and policies in the United States. The commission delivered its final report to Congress in 1976 and since then, no systematic attempt has been made to monitor sports gambling.

The last time the Justice Department attempted to estimate the level of illegal sports betting was in 1974, when it pegged the dollar amount of illegal sports betting in 1972 at $29 to $39 billion.

Sports gambling has increased significantly since then. Inevitably, with the increase in betting, there has been an increasing concern that gambling will threaten the character and, ultimately, the integrity of professional and amateur sports.

Says Tex Schramm, president of the Dallas Cowboys, "The danger from a sports standpoint is that there is always the possibility that someone might try to do something that would affect the outcome of the game. But the federal and state governments seem to put a higher priority on stopping things like illegal drug traffic. They must feel that gambling doesn't pose the problem to our society that other things do."

Lawrence O'Brien, commissioner of the National Basketball Association, observes that "even a suggestion of scandal would be devastating to our sport," and National Football League Commissioner Pete Rozelle has warned that sports betting poses the threat of creating a "generation of cynical fans obsessed with point spreads and . . . constant suspicion of the motives of players and coaches alike."

The NFL, the NBA and the other professional sports leagues maintain security staffs with representatives in every franchise city to keep an eye out for any contact between gamblers and the teams. "We follow the point spreads on all our games, and any time there is a change in the point spread we investigate," says Jack Joyce, who directs security for the NBA. "Our security representatives in each of our 23 franchise cities are usually former law enforcement people. They know who the gamblers are and they know the law enforcement people."

It is probably one of the supreme ironies of the sports-gambling phenomenon that while all the professional sports leagues and amateur sports associations oppose sports betting in any form, the gambling and sports industries also share some vital interests.

"You have a strange alliance between the people who are involved in professional sports and the people who are involved in betting because they are interested in an honest game and we are interested in an honest game," Schramm says. "The public that wants to bet on sports has an interest in the honesty and integrity of the sport they are betting on."

Some maintain that betting is at least partially responsible for the popularity of professional sports--a notion that is disputed by the professional sports commissioners.

"I happen to believe the tremendous popularity of the National Football League is because there are so many people wagering on it," says Austin in Las Vegas. "There just would not be such a tremendous interest in sports if there were not so many people betting."

The volume of sports betting will increase as more sports contests are available on cable television, Austin says. "There will be more money wagered because people will want to become rooters, even if it is not for the home team."

Since quitting his job in California and moving to Las Vegas, Austin devotes full time to his gambling activities, rising as early as 4 a.m. to pore over voluminous statistics and charts and contact a network of sources around the nation before placing his bets.

"There is no system that will make for winners consistently," says Austin. "The only system is good, hard work, although I do have a lot of sources around the country who will tell me about injuries." He will not reveal his sources, but says they are people with access to the teams.

For any gambler, winning consistently is impossible. Consider the case of Harry--not his real name--a veteran of 15 years of serious sports betting in the Washington area.

The proprietor of a small business in Northern Virginia, Harry has had good years and bad ones in his gambling career. "Overall, there's no question about it. I have lost money. But I know I'll never stop," he says.

"As a sports fan, you have opinions, and it's only natural that you would bet them. When you bet on a game, you are purchasing a thrill you can get in no other way. It's a three-hour high."

Currently, Harry is betting with three bookies, none of whom has ever been arrested. All say they would probably get out of the business if they were arrested, Harry says, because they don't want to take the chance on a lengthy jail sentence that would most likely accompany a second offense.

"I know one guy who only took bets on football. He made $60,000 in one season, and he paid no taxes on it. Most of the bookies are honorable people. They'd be out of business if they didn't keep their word. I'd trust them ahead of some of the people I've met in the business world," Harry says.

Currently, Harry's bets average $200; he may bet four or five games a weekend. Like most bettors, his favorite game is football, although he occasionally bets basketball.

"I can't think of any better way to spend New Year's Day than to get in some food, have some friends over, have bets on all the bowl games and then watch them on television," he says.

Harry says he can remember a time when he bet $30,000 a week on sports contests, but he's cut way back now and he says his greatest fear is that his bookmakers will cut him off because they don't get enough action from him.

"When you bet real big money, you tend to lose perspective," Harry says. "In my case, I was neglecting my business. When you've just won $10,000 it's hard to get excited if one of your employes spends four hours too long on a job."

Horowitz, a Long Island-based sports handicapper, believes that true bettors undergo a personality change when watching a contest they have wagered on; instead of being spectators, they imagine themselves participants.

"It transports you for three hours into a fantasy world," Horowitz says.

It has long been a matter of debate just how many sports bettors are serious gamblers--as opposed to social bettors or office pool participants. NFL Commissioner Rozelle says he has been told by law enforcement officials that no more than 1 percent of the fans are serious gamblers.

Schramm of the Cowboys says those who bet seriously make up only a small percentage.

"Of course through television we are reaching a lot of people. And a small percentage of a lot of people is still a lot of people."