Police on scene drew their weapons when the object was discovered during a security check.
Duty Sgt. Paul Wozney says there was no altercation and the suspect is being charged with possessing a weapon dangerous to the public peace.
Last week, a protest at city hall in support of Palestinians who have been killed by Israeli fire in Gaza turned violent when a handful of Israeli supporters showed up.
Some men began pushing and shoving, and one man stomped on an Israeli flag before police moved in and set up a line of officers to separate the groups and try to ease tensions.
On Wednesday, Wozney said the man who was taken to custody appears to have acted alone and was not related to last week's clash.
Calgary police had a stronger presence at Wednesday's event after admitting there should have been more officers at the city hall protest.
Spokesman Kevin Brookwell said the department's Major Event Emergency Management Section, which not only monitors, but interacts with protest groups, didn’t place the call to deploy more officers.
“Previous protests by the groups involved have been peaceful,” he said.
“It’s also possible outsiders are to blame for the violence. It’s not uncommon for people not affiliated with a protest to swoop in, cause trouble and leave.”
He said police are continuing to follow up on complaints from people who were at the city hall protest who claim they were hurt.
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