At a time when Muslims in the US are facing scorn and bigotry, the boxing legend is remembered as the face of 'Islam'.
Muhammad Ali's funeral will be held on Friday.
The interfaith service will be led by Imam Zaid Shakir.
Former US President Bill Clinton, journalist Bryant Gumbel and actor Billy Crystal will be among those giving eulogies.
At a time when Muslims in America are facing scorn and bigotry, the late boxing legend Muhammad Ali should be remembered as the true, peaceful face of Islam, residents of his hometown say.
Hundreds of people filed past his childhood home in Louisville on Sunday, now a museum dedicated to his remarkable life, to honour the three-time heavyweight champion known simply as "The Greatest".
Mourners left flowers and other mementos remembered his sporting prowess and his activism, but also spoke of Ali and his Muslim faith and how his example can help dispel stereotypes about Islam.
"With the stuff going on these days, most of the time, you see in the media there's a bad image of Muslims," said Hamza Shah, a doctor in Louisville, where Ali grew up and first started boxing.
"The one person we can definitely get a good image of was Muhammad Ali, and he portrayed what the real Islam is."
Republican presidential hopeful Donald Trump in December sparked outrage when he suggested a temporary ban on all Muslims entering the United States.
"We as Muslims have to stand up to those who use Islam to advance their own personal agenda," Ali said in a sharp rebuke to the Trump proposal.
"I believe that our political leaders should use their position to bring understanding about the religion of Islam, and clarify that these misguided murderers have perverted people's views on what Islam really is."
'Man of truth'
"He stood up as a man of truth, and Muslim countries look to people who not only are truthful but also compassionate and merciful," Chicago-based imam Syed Hussein Shaheed said.
Ali was respected throughout the Muslim world - from Pakistan to Indonesia, from Saudi Arabia to Malaysia and across Africa - for the values he espoused and promoted, the imam added.
That message of tolerance and compassion was celebrated Sunday at an interfaith prayer service in Ali's honor at Louisville's Islamic Center.
"At a time when a candidate for the most powerful position in the world encourages us to fear those who are different from us, we need the voice, we need the presence of Muhammad Ali," said Derek Penwell, who leads a Christian church in the city.
After the September 11, 2001 attacks on New York and Washington, Ali sharply cautioned Americans against categorising all Muslims as 'extremists'.
December response to Trump, saying: "True Muslims know that the ruthless violence of so-called Islamic jihadists goes against the very tenets of our religion."
Democratic presidential hopeful Bernie Sanders, who is fighting Hillary Clinton for the right to face Trump in November, said Saturday that Ali was not only an elite athlete but a champion of civil rights, and a true believer in Islam.
"To all of Donald Trump's supporters who think it is appropriate to tell us that they love Muhammad Ali but they hate Muslims, understand that Muhammad Ali was a devout Muslim who took his religion very seriously," Sanders said.
Ali's funeral will be held on Friday in Louisville, and will be preceded by a public procession.
The interfaith service will be led by Imam Zaid Shakir, a close friend of Ali and co-founder of Zaytuna College, the first Muslim liberal-arts college in the US, who will lead the Muslim funeral prayer.
Former President Bill Clinton, TV journalist Bryant Gumbel and actor Billy Crystal will be among those giving a eulogy for Ali at the service.