Earlier, Brown's body was carried to the theater through the horses.
Hundreds of fans followed behind the caisson singing the chorus of Brown's anthem, "Say it Loud - I'm Black and I'm Proud."
To many, Brown was more than just an energetic performer. As Norman Brand of Harlem waited for the procession to begin, the his native Alabama.
"It really changed the attitude of most black people. It was like a wake-up call. Before that, if you were called black, it was like an insult," Brand said.
"Just one song and one word can change a whole situation."
Mourners came from far and wide to attend the first in a trio of life.
His casket left a Georgia funeral parlor Wednesday for an all-night drive to New York.
It arrived at Sharpton's Harlem headquarters just before noon Thursday, and was quickly transferred to the carriage for a 20-block procession to the theater.
Sharpton accompanied the body from Georgia and walked behind the carriage Thursday. He stood at Brown's side for hours during the viewing.
On Friday, a private ceremony is planned at a church near Augusta, Ga. A second public viewing of the singer's body will be held Saturday at the James Brown Arena in Augusta.
Some fans arrived at the Apollo as early as midnight for a chance to pay their respects.
More than 100 people were in line outside the theater by 8 a.m., drawn to a man who left an indelible mark on soul, R B, funk, disco and rap music.
Later, the crowd swelled into the thousands and spilled over onto both sides of 125th Street. The line to get inside the Apollo stretched for blocks.
"He seemed like family, a friend of mine," said Brenda Harper, who was the first to arrive, shortly after midnight.
Fourteen years ago, she said: "I jumped on the stage and he danced with me. I danced with the Godfather that day."
Musicians and celebrities slipped in to pay their respects throughout the day: boxer Joe Frazier, band members including bass player Fred Thomas, and Ali-Ollie Woodson, who was a singer with the Temptations in the 1980s and again in the early 1990s.
Relatives passed through, too, some wiping away tears.
"He was my uncle, but he acted like a big brother to me," said Brown's nephew Earl Swindell, 54, who acted as a pallbearer. "I loved him, though.
I was right there with him till the end. He meant a lot to me."
Brown, who lived in Beech Island, S.
C., continued to work to the end, dying less than a week before he was to perform New Year's Eve in Manhattan at B.B.
King's blues club. Chaka Khan, the Grammy Award-winning rhythm and blues performer, will play instead.
He had also talked recently about returning to Harlem, friends said.
"He told me two weeks ago to book the Apollo for two days," said his friend and manager, Charles Bobbit. "He said, `Let's play two days at the Apollo, and we'll see the lines again around the block."'
"The Apollo was always his home because that's where it all started," said his agent, Frank Copsidas, "and the people of Harlem were his family.
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(Associated Press Writers Marcus Franklin and Adam Goldman contributed to this report.)