Miami's 20,000 capacity stadium was close to full despite a huge thunder storm just before the event to celebrate the arrival of the Argentine World Cup winner was due to begin.

A massive tropical downpour forced organizers to delay the start of "The Unveil" as fans took cover or waited out the rain in their cars.

The event started around two hours later than originally scheduled with wet but delighted fans chanting Messi's name as the lights dimmed on the venue.

The club's co-owner David Beckham opened the ceremony saying Messi's arrival was a "dream come true".

"Leo, we are so proud that you have chosen our club for the next stage in your career," said the former Manchester United player whose move to MLS's Los Angeles Galaxy in 2007 was the league's previous biggest arrival.

"Welcome to our family Leo," Beckham added in Spanish, to roars from the crowd.

Miami's majority owner Jorge Mas then whipped the crowd up with his own bilingual speech.

"Tonight is a gift and celebration to the city that opened its arms to my family," said the Cuban-American businessman.

"Tonight we are doing this in the rain. "This is holy water!" he added.

"This is our moment! Our moment to change the football landscape in this country," he added, before introducing Messi as "Your new number 10, America's number 10".

Messi, who arrived in Miami on Tuesday with his family and caused a viral sensation after being photographed shopping with them in a local supermarket, thanked the supporters.

"Thank you very much to all the people for supporting me and giving me this love. I am very happy to be here in Miami," he said.

"I really want to start training, to compete, I have the same desire that I always had to compete, to want to win, to help the club continue to grow.

"I am very happy to have chosen to come to play in this city with my family, to choose this project and I have no doubt that we are going to enjoy it a lot, we are going to have a good time and very good things are going to happen," he said.

Inter Miami, the bottom club in Major League Soccer, confirmed on Saturday that the seven-times Ballon d’Or winner had signed a deal that will keep him with the club until 2025.

The former Barcelona great opted to join Miami after his contract with French club Paris Saint-Germain expired last month.

His former Barcelona team-mate, Spanish midfielder Sergio Busquets, was also presented to the crowd after completing his move.

Major League Soccer commissioner Don Garber praised Mas for his determined pursuit of Messi for the club which is in only its fourth season.

- Greatest of all time -

"Lionel Messi was on his radar from the very beginning. I have to say, when he would say that many years ago, I didn't think that he would be able to deliver it," Garber told reporters.

"But here we are today with a player that I think, without doubt, is not only a generational player, but in my opinion, the greatest of all time."

He was scheduled to hold a press conference on Monday but the club said Sunday that had been postponed. No explanation for the decision was given.

Messi could make his debut for the club on Friday against Mexico team Cruz Azul in the new Leagues Cup, which features teams from both MLS and Mexico's Liga MX.

However on Saturday, the team's new Argentine coach Gerardo Martino urged "patience" after watching his side's winless streak extend to 11 games.

"We look to the future with hope (given) that the best player in the world will play for our team, but also with the patience that we have to have so that he can get in good physical shape and can play at the right moment," he said.

Latin music stars Camilo and Ozuna were set to entertain the crowd at the DRV PNK Stadium, about an hour's drive north of downtown Miami.

The stadium, Inter Miami's temporary home on the site of the old home of the now defunct Fort Lauderdale Strikers, was not completely full as the event got under way with fireworks.

Inter are planning their own purpose-built stadium, Freedom Park, in Miami but it is not expected to open until 2025.