ON A HOT July night in Savannah La Mar, Jamaica, 600 people gather in a yard the size of a football field, dancing in the darkness.

The air is dense with smoke that smells of burning ganga (marijuana) and woodfires and the pots of corn cooking. At one end of the yard is a blockhouse, out of which black wires creep like electrical vines into the tropical heat. Inside is a Kingston deejay who has brought the music, electronic equipment and party atmosphere, that's known collectively, in island parlance, as "soundsystem."

The collected hundreds, at a cost of two Jamaican dollars each, are dancing in place near tape recorders they've brought along to plug into the monster audio rig. That is no small price in Sav La Mar, but any amount would seem reasonable for "steppin' . . . steppin' . . . steppin' out of Babylon" to the sound, philosophy and way of life that is reggae.

Reggae, the indigenous Jamaican pop music, is characterized by its heavy bass lines, distinctive melodic and rhythmic style, and a syncopated, off-the-beat drumming. Infectious and danceable, its forebears include Caribbean folk music, African rhythms, American rhythm-and-blues and a hodgepodge of lesser influences. There is similar music to be heard -- for example, in the "cadence music of the French-speaking Caribbean islands -- but reggae is utterly unique in its lyrics.

Reggae lyrics bite. And for good reason. Among their topics are the social, political, racial and economic inequalities of the Third World, not to mention the doctrines of the Rastafarians, whose quasi-religious philosophy and black nationalist politics have been an increasingly great influence on Jamaica's urban poor and unemployed.

But despite its local Caribbean concerns, reggae has been a surprising success in the U.S. market. Recording-company executives have been skeptical of reggae's potential since it was introduced to the U.S. market six years ago, when a number of critics predicted a reggae explosion. But interest is growing: Rolling Stones Records recently signed Peter Tosh away from CBS, and industry figures now show that in a period of generally declining pop-record sales, reggae is doing better than ever. I'm living in the present And I'm walking in the future Steppin' in the future . . . Got to be a Mystic Man . . . Peter Tosh, "Mystic Man"

Peter Tosh's concert in New York's Central Park two weeks ago drew a crowd of 28,000. His recent album, "Bush Doctor," sold 100,000 copies in eight months. And his new album, "Mystic Man," released less than two months ago, reportedly has already sold 70,000 copies, and the re-order numbers are good.

Backstage at the Warner Theater last Tuesday, Tosh was wearing a pair of very dark sunglasses and a green knit cap which covered his dreadlocks. He looked pleasantly dazed: perhaps because of what he was smoking, but certainly because of the success of his tour.

Asked why the reggae sales figures are rising now -- six years after reggae was supposed to take off -- Tosh was characteristically oblique.

"Because," he answered, smiling, "generally what people predict generally happens. And my father say he will work mysterious things, his wonders to perform." He removed another veil from his message: " 'The stones that the builders refuse will become the head cornerstones.' So it is just a fulfillment of prophecy, you see."

Tosh waited a minute and then decided to try a more direct answer. "Reggae," he said, "is solid music. And because of the obstacles and barriers in the way, it had difficulties in reaching the peak of its recognition."

Barriers?

"Well, the barriers were -- or are -- 'medi-otic' obstacles, controlled by the media. Those who honcho the press that prints propaganda and public mischiefs, and those who honcho the radio stations that will not play the music, those are major media obstacles. Nothing is being said or being published good about reggae music. It is being said to be too political every time. They generally say that."

But doesn't it seem to be overcoming some of those obstacles?

Tosh grins, obviously about to enunciate some cosmic Rasta perception: "Yes, you know, because righteousness can stay forever. And, I have already said, the stones that the builder rejected . . . "

Gotcha.

Tosh is a Jamaican national hero, as well as a rising pop star -- why the popularity?

"Because I am powerful?," he asks. "No. You see, in the beginning was the word and the word was with JAH [god]. The same in this time, with those who know JAH, you see, and trust in JAH. I am not sure that I am powerful. What is the power of the father in me that is awful. And the most thing that makes it look as if I am powerful is because I speak the truth in a world of fantasy and illusion. But wherever the truth is being spoken, in this world of Walt Disney, it is torn down."

Tasta philosophy involves a belief in JAH, the restorative powers of marijuana, the practice of good works, and natural food, clothing and work implements. It is not a religion, adherents say, but a traditional way of life.

Politically, the Rastas are more specific. As their manifesto states: "The Rastafarian Movement has as its chief aim the complete destruction of all vestiges of white supremacy in Jamaica, thereby putting an end to economic exploitation and the social degradation of the black people." Thus it is not unlike other black nationalist movements -- except for its musical platform. By the rivers of Babylon Where we sat down And where we wept When we remembered Zion . . . "By the Rivers of Babylon"

The Rastafarians label anything which is bad as "Babylon." Rum is Babylon. Jail is Babylon. Slums, poverty, unemployment: All are Babylon. So are oppression, discrimination and the unequal distribution of money, rights and justice.

The Rasta vocabulary, with its mix of biblical allusion and political rhetoric always confounds the uninitiated. But Jamaicans have learned it through reggae, which has appropriated Rasta language and philosophy, and distributed it ecumenically among the populace.

In a country where electricity is scarce, and television a rarity, transistor radios and tape players abound. And although one of the undisputed successes of the Michael Manley government is its literacy program, radio remains the medium which has the greatest impact on Jamaican society. RJR and JBC, the island's two radio stations, broadcast a little news and a lot of music. Those who don't want to get the word from the BBC can get it from reggae giants Bob Marley, Burning Spear or Peter Tosh. When their music is too controversial for broadcast -- not a rare occurrence -- it is available at the record store. But not for long: Banned music sells out fast. Music and Herb is the healing of the nation Sent from JAH, who we pray So take ye heed and listen, listen Be a fan for better days, better days Light your spliff, light your chalice We gonna smoke it in Buk-In-Hamm Palace . . . Peter Tosh, "Buk-In-Hamm Palace"

Marijuana is illegal in Jamaica -- and that law appears time after time in the music of Marley and Tosh. In Jamaica, a lush land where unemployment hovers constantly around 30 per cent and in 1978 the inflation rate hit 59 per cent, they feel that marijuana is a big issue. So when they write song after song about legalizing marijuana, they have in mind fellow Jamaicans for whom a grass-based economy would mean relief from grinding poverty. And they're urging Americans, too, to "legalize it -- don't criticize it." That is because legalization in Jamaica couldn't mean real prosperity as long as the U.S. market is closed.

There was not much marijuana last week at Peter Tosh's concert, but there was a great deal of ecumenical feeling, as well as local proof of Tosh's belief that his music is for "black and white, rich and poor." The audience approached the stage to watch him more closely. He didn't give any of the signals that rock stars frequently do by way of inviting a rush. And nobody rushed along the wide aisles, no pushing for a vantage point.

Two teen-age boys who looked like they had $20 haircuts and charge accounts at Britches were watching from about 10 rows back. As Tosh finished each perfectly executed number, the band cooking along, the two boys silently raised clenched fist, waving their bent arms in the air. Tosh couldn't see them, but they weren't trying to be seen. They were just paying him a compliment.

Rich and poor alike. Mysterious ways, wonders to perform.