Billy Graham, the most famous US Christian evangelist, has been speaking to followers during a three-day mass prayer rally which may be his last.
Mr Clinton praised Mr Graham for always living by his faith
More than 80,000 people came to pay tribute to the preacher in New York on Saturday, the second of the three days - 20,000 more than on Friday.
Correspondents say Mr Graham, who is 86 and has prostate cancer, was upbeat.
He was greeted on stage by former US President Bill Clinton, who hailed him as "the man I love".
Speaking briefly before Mr Graham's own sermon, he described how the preacher had refused to speak to a segregated audience in Mr Clinton's home state, Arkansas.
"I was just a little boy and I'll never forget it," he said. "I've loved him ever since. God bless you, friend."
Mr Graham described Mr Clinton and his wife Hilary, who also attended, as "wonderful friends" and a "great couple".
The veteran preacher spoke for around 15 minutes, as temperatures soared at the Flushing Meadows venue in the city's Queens district.
Thousands also watched him on giant screens erected outside the park.
He addressed young people, with a parable about bad decision-making taken from the plot of the latest Star Wars film.
The third of his three sermons, which is expected to be his last in the US, comes on Sunday.
He is considering a request to hold a rally in London in November, but is said to find time differences and separation from his sick wife Ruth problematic.
Billy Graham is thought to have preached to more people around the world than the late Pope John Paul II.
Speaking on Friday, he urged the audience to put their faith in Jesus and show love, not hate.
He looked frail as he spoke to the crowds, but made light of his afflictions, which include cancer, fluid on the brain and Parkinson's disease.
Billy Graham's son, Franklin Graham, who is poised to take over his father's ministry, caused controversy shortly after the 11 September 2001 terror attacks by describing Islam as a "wicked and violent" religion.
The BBC's religious affairs correspondent Jane Little, in New York, says that while other preachers have got involved in politics, Billy Graham has tried to focus on spreading the gospel.