'Iron Boy' saves Sydney as Downey Jr tweets support

Sydney: A seriously ill Australian boy on Thursday was granted his wish to become a superhero like Iron Man, with police staging an elaborate event on the Sydney Opera House steps as Hollywood star Robert Downey Jr tweeted his support.

Nine-year-old Domenic Pace, who has battled cystic fibrosis throughout his life, has long idolised Marvel superhero Iron Man and expressed his wish to be "Iron Boy", with the Make-A-Wish Foundation granted his dream to fight crime in Australia's biggest city.

In scenes reminiscent of Batkid a five-year-old boy recovering from cancer who in 2013 became the diminutive version of the crime-fighter Batman in San Francisco Sydneysiders turned out to watch Pace save the day on the harbour in a spectacle that took almost a year to organise.

"Sent a very special boy on a top secret mission today. Go get 'em, Domenic! #IronBoyAU @MakeAWishAust," American actor Robert Downey Jr, who plays Iron Man in the Marvel films, tweeted Thursday.

New South Wales Police Commissioner Andrew Scipione told Pace, who was playing in a Sydney park when he was flown by helicopter to police headquarters for the surprise mission, that authorities needed his help.

"Make-A-Wish reporter Hope Joy has been kidnapped. Officers have confirmed that Clark Island has been overtaken by Ultron's henchmen and Hope Joy is being held captive," Scipione said in a video recording.

"If I could make one wish, it would be for you to help us, Iron Boy."

Pace was fitted with a specially made Iron Boy suit and his 12-year-old brother Joseph played the role of Iron Man sidekick Colonel James Rhodes as he told reporters he was "ready to get down to business".

The schoolboy was then whisked off in an inflatable boat to Clark Island in Sydney Harbour, where he freed the fictional reporter before engaging in a fight with villainous robot Ultron on the steps of the iconic Opera House.

"Go Dom!," Australian thespian Liam Hemsworth, the younger brother of Hollywood star Chris Hemsworth who plays Thor in the same series of movies, tweeted as #IronBoyAU became the top-trending hashtag on Twitter in Australia.