Calgary Mayor Naheed Nenshi says there were threats against a nude swimming event that prompted the city to cancel it, including a bomb threat and the claim that licence plate information would be collected in the parking lot.

"Some of the folks, both online and in phone calls to city staff and public servants, were threatening some really unpleasant stuff," said Nenshi on CBC's Alberta@Noon on Friday.

"There was a bomb threat. There were people who were saying we're going to write down all the licence plate numbers and post them as pedophiles; we're going to take a baseball bat to car windows. This is obviously incredibly unacceptable behaviour, and it's not behaviour that we can ever countenance."

The nude swim event planned for the Southland Leisure Centre garnered significant attention after it was reported in the media. A petition against the event had almost 22,000 signatures, and a petition against that petition received almost 7,000 signatures.

'Nothing to do with nudity'

Nenshi said he's disappointed in the cancellation, but noted the city was unable to establish a proper security response to the threats in such a short time and pointed out there were other bookings and a hockey tournament going on in the building at the same time as the planned swim.

"It has nothing to do with nudity, it has nothing to do with petitions. It has everything to do with ensuring that everyone is safe, and it certainly is not a decision that will affect future bookings at this time," he said.

Nenshi said he doesn't tell police what to do, but he knows the service takes threats of this nature seriously and will be investigating.

Nenshi also said the city has been hosting "naturist" events for "many, many years" without issue. So has Edmonton, and naturists there are extending an official invitation to their counterparts in Calgary to go skinny-dipping in Alberta's capital instead.

The organization behind the event, Calgary Nude Recreation, has started a crowdfunding campaign to raise money for a legal challenge against the city for the cancellation.