Beneath the long reflecting pool shimmering in the park directly opposite San Francisco's City Hall is a cavernous, publicly owned convention facility known as brooks Hall. Some weekends it's home for furniture shows, boat shows, or exhibits of alternative energy products. This weekend it became a massive head shop.

The civic center was playing host to purveyors of drug paraphernalia -- the assorted pipes, rolling papers, mirrors and what-all that go with the use of marijuana and cocaine.

The event, the Marijuana Reform Festival, was billed as the nation's first trade show of products aimed at a massive, illegal market known to exist, but impossible to measure.

A three-ring circus of lawyers, rock bands, comedians, psychiatrists, campy 1930s antidrug films and natural foods (soya milk ice cream and "organic burritos") was pulled together for the show.

We're trying to educate people," said 43-year-old organizer Stan Politi, "and we recognize that we have to do that in an atmosphere of fun. We're trying to get people with families. If we don't get them, then I'd say we've failed."

Politi feared that his high hopes were going up in smoke after sparse crowds attended Friday night. By midafternoon Saturday, however, some of the Bay area's top bands were blasting music through the underground display hall and customers were lining up. By Sunday, at least 15,000, mostly white, lower middle class couples, had paid $4 each to get in. A portion of the proceeds went ot NORML, the National Organization for the Reform of Marijuana Laws, which seeks to legalize cannabis.

More than 200 entrepreneurs, many of them wholesalers, came pushing pipes made of bone, wood, ivory, brass, glass, antler, stone and ceramic. Some were mass produced; others, like a three-foot-long carved pipe with a goat's skull bowl, were exquisitely crafted. Also featured were "tooters," tiny tubes for snorting cocaine, along with kits containing mirrors, tiny spoons, and gold-plated razor blades for dividing the drug into thin lines to be inhaled.

These products are legal, but the drugs to be used with them are not, so local drug lawyers gave $1 consultations and "Herb the Herb Man," recently arrested for possession of marijuan, collected money to pay his fine.

Also collecting money were the Hell's Angels, now the targets of a massive drug-conspiracy federal court case. They were selling bumper stickers in the shape of an American flag reading "Help Keep America Free -- Support Your Local Hell's Angels."

With each donation came a photo montage poster from the '60s, showing, among other things, an Angel about to tear a frog in two.

"People don't believe the Angels have changed like everybody else," said Dana, a 20-year-old brunette at the booth who refused to give her last name. "Everyone was radical then, but they mellowed out."

Elsewhere, a big seller, at $9.95, was "Legal First Aid for a High Society," which its author calls "the book 30 million people have been waiting for."

Kenneth Weiss, a young, bearded author-attorney, says he got that figure from the magazine polls, and is inclined to believe it. Using only ads in a drug-related publication, Weiss says he has sold 15,000 copies in just three months.

Marijuana is said to be among the top 10 cash crops in California. Nationwide the amount of money spent on marijuana has been estimated as high as $48 billion, a figure arrived at by adding the estimates of the money that passes hands from grower to smoker -- a sort of Gross National Drug Product. That would rank it behind oil and automobiles. Stan Politi, 43, the festival organizer, uses the figure publicly but privately disputes it.

"How could it be?" he asks. "The combined figure for alcohol and tobacco is $46 billion. But it's the government's figures, so we use them."

Politi says he does not smoke dope himself, but he has a teen-age daughter who does. He is concerned, however, that she may be doing it to excess.

Whatever the national marijuana sales figure may be, individual dealers are clearly doing well.

Eric Davis, 26, is a partner in "Products of Distinction," which markets "Dial-a-Toke," a phony telephone that blows cooled marijuana smoke through the mouthpiece. It sells for $250. His umbrellas, canes and golf clubs, with pipes carefully carved into the handles, are wholesaled for $25. Davis says he has sold 300,000. The company has offices in Los Angeles, Chicago and San Francisco.

Christine Joseph 30, a business graduate of George Washington University, runs a jewelry and paraphernalia business in Mt. View, Calif., with 28 employes. Her products, delicately sculpted in silver and ivory, retail for $80 and up and were never market tested. "I just took them to the San Francisco gift show in 1975," she says, "and they just took off."

Stuart Dvornin, 36, finished Georgetown University Law School and was a trial attorney for the government. Now he sells "Hydropot," a homegrowing kit designed especially for closets. He says he has sold almost 40,000 at $50 each.

Looking over the displays was Willa Elam, a 36-year-old woman, mother of two teen-agers. She had come, she said, "because I'm into this. What can I say?"

She was alone. "I'm a military housewife," she explained. "My husband is a 20-year Navy man. I'm straight by day. He wouldn't come here. The kids could come, I want them educated, but my husband would go through the roof."

Beverly Anderson of Seattle identified herself as a tourist who "just stopped by."

"I think this is historical," she said, "and I wouldn't want to miss something that might be in the history books."