Tourists, picknickers and early arrivals for the annual July 4 fireworks display swarmed into Washington's monument areas yesterday in generally peaceful coexistence with hundreds of Yippies demonstrating against marijuana laws.

It was a cool, gray afternoon characterized by the distinctive smells of the picknickers' fires and the demonstrators cigarettes, by the sounds of visitors' radios and of the Yippies' chanted slogans.

The moment of greatest tension appeared to come about 3 p.m. with about 3,000 Yippies - by police estimate - assembled in Lafayette Park and at least 75 U.S. Park policeen lined up shoulder-to-shoulder to face them on Pennsylvania Avenue in front of the White House.

A hail of bricks, bottles and firecrackers flew out of the crowd, to ward the line of police. The firecrackers appeared to cause some apprehension among the horses of the Park Police mounted units, but the rain of missiles soon subsided with few if any hits.

By evening, Park Police said they had arrested nine adults and at least one juvenile, most on disorderly conduct charges. The demonstration for the most part appeared to be marked by sometimes uncommon but generally orderly conduct.

Using an erratic sound system, speakers who appeared earnest but were mostly inaudible delivered occasional messages on politically connected topics to an often-languid crowd that lounged about the square in blue jeans, headbands and T-shirts.

According to spokesmen for the demonstrators, the main purposes for the four-day weekend of marches and protests were to call for decriminalization of marijuana and an end to government-supported spraying of Mexican marijuana with the toxic herbicide Paraquat.

A few of the demonstrators wedged themselves onto precarous perches amid the limbs of the statues of the Revolutionary War heroes at the corners of the park and on the statue of President Andrew Jackson at its center.

Others strolled about carrying black flags adorned with a red marijuana lead emblem, at times joining in such chants as, "Free the heads (marijuana smokers) and jail the feds (government agents and officials.)"

There was considerable olfactory evidence that at least some in the crowd were smoking marijuana, but the police made no issue of it. "It's mostly small stuff," said one Park Police official.

After a few hours in the square across from an empty White House - President Carter was at Camp David - the demonstrators returned to the Lincoln Memorial area where they had made a headquarters of sorts.

Although thousands of tourists visited the memorial yesterday, according to National Park Service technician John Lee, there were no complaints about demonstrators. "It's been quite peaceful," he said.