The latest in a series of impromptu rides announced by Lance Armstrong via Twitter saw more than 5,000 cyclists bring part of Adelaide to a near standstill.
Even Armstrong, who has organised similar rides in Los Angeles, Glasgow and Dublin, was staggered by the result of his Tweet.
"I personally don't think that anyone will top the turnout we had today. That was a huge amount of people," Armstrong told Australian journalists.
"The field stretched for kilometres and kilometres ... when we turned back there was just streams of people."
Riders gathered at Wigley Reserve outside Adelaide, venue for next week's Tour Down Under, and followed Armstrong and McEwen along a coastal road to Semaphore Park and back again, a distance of about 19 miles. A simple message brought them there, posted on Twitter at 5.24pm on January 13 - “OK Adelaide ... 9am Saturday. Wigley Reserve. Come one come all!”.
"It was chaos, but controlled chaos. It's certainly the biggest one that I've been a part of," said Armstrong. "It was cool. Everybody had a good time and more or less stayed safe."
Inevitably, there were a few collisons resulting in minor injuries, but nothing that gave the local police too many headaches.
"There was a lot of cyclists and it's difficult to control when they're in a bunch that big," police traffic coordinator Harry McCallum said.
"We saw a couple of people that had fallen off their bicycles but there was nothing that looked serious," he added.
Armstrong, 38, will seek an eighth Tour de France title in July after coming out of retirement last year.
"I'm just surprised by the sheer numbers, this is out of control," McEwen said.