NEW YORK, JUNE 21 -- Before the Harlem Boys Choir sang, tonight's gathering at Yankee Stadium had taken on the look of a very politically correct rally, complete with African garb and banners.

But when the red-jacketed youngsters began singing "Nkosi Sikelel' i Afrika," the haunting anthem of the African National Congress (ANC), the milling crowd stilled, and several thousand attendees rose, arms outstretched and fists clenched.

By the time singer Melba Moore had followed with "The Star-Spangled Banner" and "Lift Every Voice and Sing," traditionally known as the Negro national anthem, the spectators had bonded, holding hands and swaying. The stage had been set for Nelson Mandela.

The stadium contained more than 40,000 people and represented a setting generally familiar to Mandela, who has given several speeches in South African soccer stadiums since his release last February after 27 years in prison.

"We are truly among our own brothers and sisters," Mandela told the crowd. "We feel at home." He was greeted warmly with a three-minute standing ovation punctuated by chants of "Amandla," which means "power" in Zulu.

Wearing eyeglasses and speaking in his deliberate style, Mandela thanked New Yorkers for a warm reception that he said "reflects the strong feelings of kinship you feel towards us and the just cause we are fighting for."

Mandela sounded grateful, determined, defiant and optimistic. "Victory is in sight," he said, referring to blacks' struggle against apartheid. "We ask you to walk this last mile with us. . . . Nothing will stop our date with destiny."

He appealed for donations to help rebuild the ANC, repatriate South African exiles and educate young South Africans at ANC installations in Lusaka, Zambia.

After his remarks, Mandela traded his somber gray suit jacket for a navy-blue, satin Yankees baseball jacket and billed cap.

"You now know who I am," he told the cheering crowd with a broad smile. "I am a Yankee."

As at other events for Mandela in his first two days here, most members of the crowd were black.

Most wore some combination of the ubiquitous T-shirts and buttons so familiar at Mandela's appearances here. Traditional African kofi hats and kente cloth scarves were everywhere, but more than a few participants wore shirts decorated with the television cartoon character Bart Simpson -- in blackface.

Some flew ANC flags in their caps or hair. Others carried binoculars.

Periodically, the crowd burst spontaneously into chants of "Keep the pressure on," a phrase that has emerged as a theme this week.

"Together with you, we have made government listen," said Mandela, who spoke for 20 minutes. "Yes, we have broken the walls of the South African jails."

A sour moment came when New York Mayor David N. Dinkins mentioned New York Yankees owner George Steinbrenner, who initially expressed concern that spectators might damage the stadium outfield.

But boos that greeted mention of Steinbrenner's name turned to muted cheers when Dinkins announced that Steinbrenner had agreed to cover the $90,000 cost of renting the stadium. Singer Billy Joel, scheduled to give concerts in the stadium Friday and Saturday nights, donated equipment and the stage for tonight's event.

Earlier, during lulls between acts, spectators amused themselves by standing and raising yellow placards saying "Free South Africa" while doing a version of "the wave," a salute popularized at sporting events.

The Mighty Sparrow, a calypso singer, roused the crowd into dancing and stomping with a protest song about South Africa. "Don't ease up, don't ease up, keep up the pressure," he and his band urged, calling apartheid's architects "sucking vipers."

Folk singer Tracy Chapman had a calming effect, singing her current hit "Freedom Now," which she dedicated to Mandela. "We must continue to honor him, to continue to fight the struggle until all South Africa is free," Chapman said.

Also appearing were Sweet Honey in the Rock, an a cappella group from Washington; Richie Havens; Judy Collins; Johnny Clegg, and Sabuka, a South African band.

The stage, dominated by a colorful mural, was erected in front of the bleachers in left-center field, and fencing surrounded the infield area. An unusual felt-like carpeting covered the grassy outfield, and hundreds of police officers ringed the playing field.

Outside the stadium and earlier in the day along 125th Street in Harlem, vendors conducted a brisk business in Mandela paraphernalia.

"Mandela buttons, check it out, check it out, one dollar, one dollar," one teenager screamed through a megaphone in Harlem.

A woman passerby muttered: "I wonder if he knows what he's selling, or the reason behind it. I wonder if his parents told him."