The two men who preceded Barack Obama in the White House admitted Friday they had no idea the United States was implementing a new rule Monday that would require Canadians and Americans to have passports to cross the border.

George W. Bush and Bill Clinton were caught off guard during a 90-minute joint appearance in Toronto -- the first ever by the two -- when moderator Frank McKenna, the former Canadian ambassador to the U.S., spoke about how Canadians feel slighted by the new rule.

"I'll be frank with you Frank, I don't know about the passport issue," Bush told the crowd of 6,000.

"I thought we were making good progress on using a driver's licence to cross the border. What happened to the easy-pass card?"

Clinton too said he'd only heard about the passport requirement a day earlier, adding that in all likelihood most Americans were completely unaware of it as well.

That could mean far fewer Americans coming to Canada, which would be bad for the economy and our relationship, warned Clinton.

"I promise you, you have got my attention with this, so I'm going back home I'll see if there is anything else I can do," he said to cheers from the audience.

"I'd like to hear Homeland Security's defence of the passport issue."

The two leaders were in top form throughout the event, and despite the fact one is a Republican and the other a Democrat, they clearly got along and appeared to genuinely like each other.

Instead of some partisan friction, it appeared Clinton has become so close to Bush's father, another former commander-in-chief, through their humanitarian work that Bush Jr. jokingly characterized Friday's pairing as a family reunion of sorts.

"Mother said president Clinton and father share the stage so much, he's like a son to her," Bush said as he turned to Clinton.

"So brother, it's good to see you."

While both men received enthusiastic receptions from the audience at the cavernous Metro Toronto Convention Centre, hundreds of protesters gathered outside to denounce the former presidents as war criminals.

The demonstrators aped the antics of an Iraqi journalist who threw his shoes at Bush during a news conference in Baghdad, flinging flip flops and old boots at a poster with a painting of Bush in an orange prison jump suit.

"Jail George Bush," some chanted, while others yelled, "George Bush we know you, we will throw a shoe at you."

The group added Prime Minister Stephen Harper's name to their list of war criminals for the conflicts in Iraq and Afghanistan.

Many of them used the event as a platform to denounce the current government for its treatment of Omar Khadr.

Patricia Malloy, 48, of the War Resisters Support Campaign gripped a sign of Bush, declaring him a criminal. She said, although both men no longer held power, she wanted the former presidents to be held accountable for their actions.

"They could legally be charged with war crimes and their legacy lives on in Iraq, in Afghanistan," Malloy said.

Anthony J. Hall, a professor from the University of Lethbridge rallied the crowd by bursting out into raucous chants.

"There's a growing movement that is fed up with rampant criminality," Hall shouted, waving his arms in the air and shaking with frustration.

"Canadian people and people around the world are fed up with the violations of the rule of law," he yelled.

The noisy demonstrators could barely be heard inside.

Bill Mahoney, 60, a retired steelworker, said he was resigned to the fact the protest would make little difference to the former leaders shut inside, but said it was still important to show people the activist spirit.

"We need to get people talking, to get people talking about the double standard where people in power can commit mass murder and get away with it."

Inside, Clinton told the audience he's thankful for Canada's commitment to the Afghan mission.

"America has never had a better friend or ally," he said.

In what was billed as a "conversation" between the two, Bush and Clinton made separate addresses to the crowd to begin, with Clinton going twice as long as Bush and speaking about many more issues, while Bush spent most of his time cracking jokes.

"President Clinton and I used to believe in free speech," said Bush, with the appropriate pause for laughs.

"So thanks very much for coming -- we are glad you're here."

Another: "I'm writing a book. I frankly have to have something to do," he quipped. "Some people think I can't even read a book. Well, I'll prove them wrong."

McKenna didn't pull any punches in his questions, which focused on the war in Iraq, relations with Cuba, the crisis in Darfur, same sex marriage, gays in the military and the genocide in Rwanda, asking Clinton point blank why he didn't stop the killing.

"I have no defence (on Rwanda)," conceded Clinton.

"We didn't even have a meeting on it. It's one of two or three greatest regrets of my presidency."

Bush also spoke highly of Canada, and launched into a defence of free trade.

"America is lucky to have such a good friend on the northern border," said Bush.

"It's really important we don't put up walls and barriers between us. I'm a big free trader."

In March, hundreds of protesters in Calgary hurled insults and shoes at Bush's image outside the former president's speaking appearance.

Four demonstrators were arrested outside the downtown Calgary convention centre where Bush spoke for the first time since leaving office in January.