The pope ventured down mean streets tonight where cabdrivers and garbagemen fear to tread. In an open car, he cruised through the most forsaken quarters of Harlem and the South Bronx, past vacant lots, abandoned tenements and back alleys where drugs are forever bought and sold.

As the pontiff's motorcade moved down Adam Clayton Powell Boulevard on the way to Harlem's St. Charles Borromeo Church, an evangelist stood near a crowd of junkies, an amplifier strapped to her back, beseeching the marijuana smokers to "put that reefer down."

Undaunted by the boulevard's tattered look, the pontiff smiled and raised his arms to the cheers of several thousand black residents who came out to see him. He seemed in fine spirits when he finally reached the church on 141st Street. Indeed, it was there on the streets of Harlem, for the first time on this trip, that John Paul II burst into informal song, humming along with black children in a rendering of the gospel song "Wade in the Water."

"My friends, you brothers and sisters in Christ, this is the day the Lord has made," he said. "Through you I extend my greetings to all of black America."

From Harlem, the pope's motorcade journeyed to the South Bronx, where a crowd of 10,000 had gathered at a vacant lot strewn with rubble that will soon become a 160-unit housing project. Only hours earlier, a neighborhood priest had pointed to the spot where the pope would stand and said: "That's where the rats play soccer."

There, the pope delivered a short speech in Spanish for his predominantly Hispanic audience. Reading from a text illuminated by a flashlight held by Cardinal Terence Cooke, the pope said: "I come here because I know of the difficult conditions of your existence, because I know that your lives are marked by pain. For this reason, you deserve special attention on the part of the pope."

Although the speech was marred by a faulty sound system, the pope's short message stirred many on hand. He expressed hope that the housing project would soon become "a beautiful reality" and that everyone would be able to have "suitalbe housing."

"He had big words," said Carmen Pabom, her eyes filled with tears as the pope's motorcade sped off for Yankee Stadium. "These are tears of joy, not sadness. He made me really happy."

At the gleaming white stadium in the Bronx, the pope tonight encountered the most enthusiastic and responsive crowd yet in his tour of America. More than 80,000 people, crammed into every inch of the structure, erupted into prolonged cheers when the pope entered in his cream and blue limousine known as the pope-mobile.

As the pope made his way to the red carpeted altar to begin the mass, the noisy crowd grew silent immediately and lapsed into prayer. Moments later, the pope delivered his homily, which further developed the redistribution of wealth theme he had touched on earlier in the day at the United Nations. Wealthy nations, he said, have a responsibility to share their resources and help remove the causes of poverty.

"The poor of the United States and of the world are your brothers and sisters in Christ," he said. "You must never be content to leave them just the crumbs of your feast. You must take of your substance, and not just of your abundance, in order to help them. And you must treat them like guests at your family table."

The pope noted that America has gained "a well deserved reputation for generosity, both public and private," and called on Catholics to continue that tradition. "Make an effort," he said, "to ensure that this form of aid keeps its irreplaceable character as a fraternal and personal encounter with those who are in distress."

He went on to cite the gospel parable of the rich man and Lazarus, a story that he said had special meaning in the United States. Quoting from the gospel, he said: "Once there was a rich man who dressed in purple and linen and feeded splendidly every day. At his gate lay a beggar named Lazarus who was covered with sores. Lazarus longed to eat the scraps that fell from the rich man's table. The rich man was condemned because he did not pay attention to the other man, because he failed to take notice of Lazarus, the person who sat at his door and who longed to eat the scraps from his table."

At the end of the pope's homily, the throng erupted in overwhelming applause once again. There was John Paul alone on the stage, gesturing in one direction and then another, and with each gesture the cheering grew louder and the feeling in the stadium took on more fervor. For several minutes, 80,000 people clapped rhythmically, waved papal and Polish flags, sang, yelled and cried as a band played "Crown Him King of Kings."

Much of what the pope saw earlier tonight during his drive through New York was not what the people who live in the area see every day. In Harlem and the South Bronx, extraordinary efforts were made to brighten the neighborhoods along the motorcade route.

Last Thursday, a truckload of workers came to 141st Street, carrying buckets of bubbling asphalt with which they filled every pothole on the sidewalks and the street. Then came workmen pushing wheelbarrows filled with precisely cut squares of sod that turned a vacant lot into a bright green lawn. Sanitation crews followed, then brigades of broomsmen sweeping the dirty gutters, then men carrying water hoses that gave the street a glossy look.

"How come the pope have to come before they do stuff like this?" complained John Simms, the superintendent at 212 W. 141st St. "I think that's wrong."

Simms dug his heels into a loose patch of asphalt that covered the sidewalk. "Looka here," he said. "This cheap stuff won't last until the pope leaves. I can tell you this ain't gonna mean too much at all. It means a lot before he comes and it means a lot while he's here, but when he's gone it ain't gonna mean nothing.

It was just a few hours before the pope arrived, however, when Claire Westbury stepped out onto the ledge of her apartment across from St. Charles Borromeo to put the finishing touches on her flower and pottery arrangement. Indeed, all along this dreary street, a thousand flowers bloomed.

"I haven't been so nervous about nobody coming here since my sons came home from Vietnam," said Westbury. "And I'm not even Catholic. I just like the man and what he stands for. Here he is coming around here after all the bad things they say about us -- makes you think maybe we ain't so bad after all."

In a nearby apartment, Eunice Banks, a 60-year-old grandmother, cleaned her windows so she could have the clearest view of the pope. Her false teeth, also being cleansed for the papal visit, sat in a Mason jar next to her bed.

"The FBI been coming through here every five minutes on the lookout," she said. "Ain't nothing to worry about. Just old women and little kids over here these days. The junkies ain't gonna bother the pope unless they think he got some dope."

In the South Bronx as well the pope's visit elicited mixed feelings among the residents. Father Robert Perralla said his parishioners were proud that Pope John Paul II would come "where taxi drivers refuse to go." His feelings were seconded by flower shop owner Dora Mazzella, who said: "They're all afraid to come to the South Bronx, but the pope is coming. That's a great honor for us."

But many of the neighborhood's Hispanic, black and Italian residents complained that the interest in the South Bronx would be just a one-day thing. "They never have money for anything up here, and then the pope comes and they whitewash everything," snapped Richard Rogers, who owns a collection agency. "It's just like when [President] Carter came through here. All this money was going to come to us, but it never happened."

An abandoned building on the site where the pope stopped, for months a neighborhood eyesore, suddenly was razed in two days last week. "That," observed Joe Bugliarelli, owner of Joe's Sweet Shop across the street, "would have taken two years, normally."