A five-year-old boy with cancer has become a superhero for a day, as San Francisco transformed itself into Gotham City and thousands turned out to see the Batkid fly to the rescue.
Even President Barack Obama got in on the act after the story went viral, sending pint-size caped crusader Miles Scott a video message telling him "Way to go!".
Scott, who was diagnosed with leukaemia when he was 18 months old, was cheered as he roared out in his Batmobile to nab arch-villain Riddler and save a damsel in distress tied to the city's famous tram tracks.
Police chief Greg Suhr ordered his men to help the diminutive crime-fighter take on his nemesis the Penguin, while he also got messages from San Francisco's baseball and NFL teams for his day in the limelight.
"Our hero has arrived. The streets of San Francisco are safe today" the San Francisco 49ers tweeted, as social media went wild for the youngster, prompting huge crowds to follow him.
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The story then took off, making news around the country and overseas, with live coverage by outlets including CNN.
"I've never seen anything go viral like this, with the outpouring of support from across the world," said Patricia Wilson of Make-a-Wish Foundation, the charity which organised the dream day.
Thousands gathered in the city's Union Square when the Batkid took a break to refuel at a local restaurant.
"Even superheroes have to have lunch," explained one TV reporter, surveying the throng stretching across the square.
The city's San Francisco Chronicle printed a special edition for, with a Gotham City Chronicle masthead and a screaming headline: "Batkid Saves City!"
The US Attorney's office put out a spoof press release - datelined "San Francisco/Gotham" - announcing that the Riddler and the Penguin had been charged with conspiracy and kidnapping.
"Duo faces long prison terms thanks to Batkid," said the release from the office of the US Attorney for the Northern District of California.
In Washington DC, Obama released a message via the Vine video sharing service, looking into the camera and saying "Way to go, Miles! Way to save Gotham City."
Scott, whose leukaemia is currently in remission, was also due to be given the keys to the city by San Francisco Mayor Ed Lee, during his day as the famous comic-book hero.
The US Attorney's press release said "The Riddler" and "The Penguin" were pseudonyms for Edward "E" Nigma, and Oswald Chesterfield Cobblepot.
"We've been chasing Nigma and Cobblepot for years and just when I was about to give up hope that we would ever bring them to justice, wouldn't you know it - Batkid shows up and saves the day," said US Attorney Melinda Haag.
"I've been involved in some unbelievable cases and I've worked with some pretty remarkable law enforcement officers, but the bravery displayed by Batkid is off the charts. I'm absolutely certain that there is no villain this remarkable superhero can't defeat," she added.
