WINNIPEG -- Prime Minister Justin Trudeau offered rare praise Saturday for Conservative predecessor Stephen Harper but at least one Liberal elder statesman found it a bit hard to swallow.

Bob Rae was caught on video sticking two fingers in his mouth, pretending to gag.

The former interim Liberal leader later refused to comment on his gesture, which was caught on video by a reporter for The Canadian Press.

However later he posted an apology on Twitter calling it "a joke in poor taste that I regret very much."

The video, posted on Twitter, quickly made the rounds at the Liberal national convention, provoking both hilarity and consternation.

Rae's gag reflex was stimulated during a speech by Trudeau to the convention, the first such gathering since the Liberals defeated Harper's Conservatives in last fall's election.

After taking a few pokes at the Tories, who were simultaneously holding their own convention in Vancouver, Trudeau turned serious.

"We need to remember that even though they may be our opponents, they're not our enemies. They're our neighbours and our friends," he said.

"So, I want to take a moment. You see, there are only a handful of people alive who know what it's like to do this job ... And I can tell you, even if you weren't a fan of his politics, there can be no doubting Stephen Harper's commitment to our country."

Trudeau asked Liberals to join him in thanking Harper, who has let it be known he will retire from politics over the summer, for "his many years of public service."

Trudeau, the eldest son of former prime minister Pierre Trudeau, said he's among the few who know first hand that it's not easy to be part of a prime minister's family. So he also thanked Harper's wife, Laureen, and his children, Ben and Rachel, for "standing with him through thick and thin."

Trudeau was not all sweetness and light when it came to the Conservatives, however.

He also noted that the Tories had been debating at their convention whether to delete the party's policy on marriage being the union between one man and one woman.

"Among other things, they're debating the merits of marriage equality. In 2016. More than a decade after we made same-sex marriage legal in Canada," Trudeau said to gales of laughter.

"Well, better late than never, right? Who knows, ten years from now they might finally be willing to admit that climate change is real. Or that tax cuts for rich people don't help the middle class,. Or that government shouldn't legislate what women are allowed to wear on their heads."

The Conservatives voted Saturday to drop the sections of its policy opposing same-sex marriage.