Ali had been admitted to hospital suffering from respiratory problems before his death
Boxing legend Muhammad Ali has died at the age of 74, a spokesman for his family has confirmed.
The three-time world heavyweight champion was diagnosed with Parkinson's disease in 1984 - three years after he retired from the sport.
In the days before his death, Ali had been admitted to hospital in Phoenix, Arizona, suffering from respiratory issues.
Reports had suggested his breathing problems had been complicated by the neurological disorder, which had long impaired his speech.
Boxing stars past and present, including Mike Tyson, George Foreman and Amir Khan are among those who have paid tribute to Ali.
Spokesman Bob Gunnell said Ali's funeral will take place in his hometown of Louisville, Kentucky, on Wednesday.
"The Ali family would like to thank everyone for their thoughts, prayers, and support and asks for privacy at this time," his statement added.
News of Ali's death first emerged following a baseball game in Florida, when the Miami Marlins displayed a tribute to Ali on video screens around the stadium.
The team's president, David Samson, claimed he was told of Ali's passing by someone close to the family - but said he was unaware that the news had not been officially announced.
Ali had a personal connection to the club, and threw the first pitch at their new ballpark when it opened in 2012.
Although he had shied away from public life in recent years, Ali spoke out against Donald Trump's calls for Muslims to be banned from entering the United States.
Back in December, he urged people to "stand up to those who use Islam to advance their own personal agenda", adding: "True Muslims know that the ruthless violence of so called Islamic Jihadists goes against the very tenets of our religion".
One of the boxing legend's final public appearances was in April, when he attended a Celebrity Fight Night to benefit a Parkinson's treatment facility in his name.
Ali is survived by his fourth wife Lonnie, along with his nine children.
Many of his loved ones reportedly flew to Arizona to be by his bedside following his admission to hospital on Thursday.
Former World Boxing champion Barry McGuigan says today should be about celebrating Muhammad Ali's life.
Ali had visited Ireland a number of times, including his fight against Al Blue Lewis in Croke Park in 1972.
He returned in 2003, when he took part in the opening ceremony for the Special Olympics.
In 2009 he visited Ennis, County Clare, the birthplace of his great-grandfather, Abe Grady, who emigrated from the town to the US in the 1860s.
Thousands of people turned out to see Ali, who was made the first honorary freeman of Ennis.
President Michael D Higgins has been leading the Irish tributes to the iconic boxer, saying the news of his death has been heard with the "greatest sadness by Irish people of all generations".
In a statement, President Higgins said: "Many will remember the wit, grace and beauty he brought to boxing and some will recall his visits to Ireland.
"All over the world people also flocked to hear him offer his view on the achievement of democracy and particularly equal rights when they were so strikingly missing in some of the richest countries of the world. He brought his message of freedom and respect for people of all races to all the continents of the world".
Ali was born Cassius Marcellus Clay in Louisville, Kentucky, on 17 January 1942.
He took up boxing at the age of 12 after someone stole his bike, and within six years had become Olympic light heavyweight champion.
Clay, then 22, was the underdog when he met Sonny Liston for the world heavyweight championship in Miami on 25 February 1964.
But he won the fight when Liston failed to come out for the eighth round and then knocked him out in the first round in their rematch the following May.
Soon after winning his first championship, Clay declared his allegiance to the Nation of Islam and changed his name to Muhammad Ali.
He defended the title eight times in the next 20 months, establishing himself as one of the most exciting and talented boxers of all time.
His catchphrase, "Float like a butterfly, sting like a bee", aptly described his remarkable combination of speed and power.
Ali was stripped of his title when he refused the Vietnam draft on religious grounds and was sentenced to five years in jail.
He appealed and eventually won. But in the meantime he was out of the ring for more than two years.
In 1972, Ali fought Al Blue Lewis in Croke Park.
He also went on to win the heavyweight crown twice more, first from the big-hitting George Foreman in the "Rumble in the Jungle" in Zaire on 30 October 1974.
The following year, 1975, Ali won possibly his greatest fight, the celebrated "Thrilla in Manila", stopping 'Smokin' Joe Frazier in the 14th.
After 10 defences, he lost the title on points to Leon Spinks in February 1978, but reclaimed it in September the same year.
Ali was now 36 and decided to hang up his gloves after taking enough punishment to last several lifetimes.
He came out of retirement for another championship fight, against Larry Holmes on 2 October 1980.
Holmes knocked him out in the 11th round. Ali retired for good after losing on points to Trevor Berbick the following year.
Soon afterwards he announced he was suffering from Parkinson's disease.
Despite his poor health, he travelled the world supporting the campaign to end Third World debt and pressing for more research into Parkinson's.