Waving "High Guy is Here" placards, about 650 Yippies turned on to a balmy summer afternoon yesterday by marching down Constitution Avenue, tying up traffic and protesting the nation's marijuana laws.

While Lunchtime idlers lingered over park bench picnics and ice cream vendors did a leisurely trade, the crowd of long-haired, often shirtless demonstrators passed joints around, chanted esoteric slogans and proceeded downtown.

At Frankin Park, 13th and K streets NW, where they stopped for a lunch of cold beer, speeches and camaraderie reminiscent the 1960s, the demonstration was disrupted by a dope-snatcher.

A burly man in a yellow T-shirt grabbed a lunchbag-sized plastic sack of what one witness described as a full pound of Columbian-grown marijuana -- a favored blend worth more than $1,000 -- and ran off.

"That's him over there" yelled James Voag, 21 of Washington, N.J., who joined about 100 Yippies in pursuit of the thief. They succeeded in triping him sufficiently to make him drop the stolen stash and chased him into the Adult Education Demonstration Center across from the park. Police interceded.

"We want justice! We want justice!" chanted the protesters, waving their fists at police who refused to arrest the alleged dopesnatcher.

"You'd have to go to court and say, 'This is my stuff,' counseled Sgt. Charlie Hall of the U.S. Park Police. He put his arm around a confused Yippie and tried to explain how tangled life could become if the victim were to swear out a warrant for the theft of a substance whose possession itself was illegal. "It could get very involved," he said.

Convinced, the posse abandoned its quarry for the greener grass of the park and plopped down to turn on and cheer the speakers. Atlantan Vicki Rosenbloom, 35, a lithe, pint-sized women in a tie-dyed "Right To Harvest" "T-shirt and Calvin Klein jeans, glared from behing Bugs Bunny sunglasses as she denounced undercover narcotics agents.

"They pretend to be your friends -- and then they stab you in the back" she shouted. Roars from the crowd.

Aron Kay, 29, a nationally-reknowned pie-thrower from San Francisco, said he dreamed of the day when a "dope supermerket" would open its doors. And Ed Rosenthal, co-author of the "The Marijuana Grower's Guide," proposed exporting Albemarle County Green as a cash crop in return for Arab oil.

"All we care about is the peace and tranquility of the afternoon," sighted Sgt. Hall.

"It's a great day for getting high," said San Franciscan Keith Van Kirk, 20, who lounged shirtless in cutoff blue jeans with an eight-pack of cold Miller's and girlfriend Sue Markee, 20, of Syracuse, N.Y.

Byron Seals, 19, a student from Knoxville whose bell earring tinkled faintly as he fired up a joint, said he had started the day by taking LSD and had followed it up with several joints of reefer and a few tall cool ones.

"I'm ready to raise some hell," he grinned.

High up in their glass cubicled offices across the street, bemused DEA bureaucrats huddled with their secretaries and grinned down as protesters vilified them with obscenities they could not hear. Finally, it was time to leave and about 50 motorscooter policemen herded the Yippies along K Street like cowboys on a cattle drive.

It was all part of a flawless summer day given to frivolous as well as cosmic pursuits, a day of small pleasures heightened by holiday anticipation and cooled by a gentle breeze.

Indeed, Good Humor pushcart operator Violet King, 42, who hawked Fudgecake bars on Pennsylvania Avenue next to the White House, blame the not-too-sultry weather for only an average day of 300 anticipated ice cream bar sales. In the ice cream business, the motto is "the hotter, the better.

"One day two weeks ago, I sold 400 bars," she said. "It was 92 degrees."

Nonetheless, it was warm enough yesterday outside the Pan Am Building at 16th and K streets NW, where legal secretary Suzie Carr, 26, nibbled on the earlobe of Dave Whitrock, 27, an office manager, after a Burger King lunch they described as "cheeseburgers in paradise."

"It must be the weather," said Whiterock. CAPTION: Picture, Yippies smoke water pipe yesterday at Franklin Park demonstration. By Larry Morris -- The Washington Post