Memo to mayoral candidates: When there are microphones in the room, be careful what you say.

The few dozen voters who showed up at the Shaw Conference Centre for Tuesday's mayoral forum likely missed the feistiest exchange.

But the news cameras were switched on, so the strange encounter was saved for posterity.

Bob Ligertwood and Carla Frost became involved in a brief exchange Tuesday prior to a all-candidate mayoral forum. (CBC)

Moments before the forum opened, incumbent Don Iveson made his way down the podium table, shaking hands with opponents.

After greeting perennial also-ran Bob Ligertwood, Iveson stopped beside another challenger and asked if she was Carla Frost.

"Yes, I am," Frost responded. "Pleased to meet you."

As Iveson leaned over to shake hands with another candidate, Frost turned in her seat and said: "I'm Sweetie. Actually, I'm Sweetie, I just heard."

Frost then pointed angrily towards Ligertwood, seated three chairs down.

"If you don't apologize to me, I'm going to knock you out," she could be heard to say.

Appeals to moderator

That prompted Ligertwood to ask moderator Dave Robertson to summon security to the table.

"She just threatened me," Ligertwood said.

Then, three times, he repeated: "Can I have security over here, please?"

"She just threatened to knock me out," he said again.

Robertson pulled the two candidates aside for a minute-long discussion.

Ligertwood and Frost finally returned to their seats and the forum, ​hosted by Edmonton Elections, officially began.

With only an hour to share, each of the 11 candidates in attendance got four minutes to make their pitches to voters.

Edmonton Elections has said it will extend mayoral forums scheduled for Oct. 3 and Oct. 11 from two to three hours, starting at 6:30 p.m., to allow voters to question the candidates.

Edmontonians go to the polls on Oct. 16.