OTTAWA -- Liberal Leader Justin Trudeau wasted little time going after his political opponents in Thursday's leaders' debate, keen to shed the charge that he's not ready to lead the country.

Trudeau repeatedly interrupted and badgered Prime Minister Stephen Harper, challenging the Conservative leader's facts, and attacked NDP Leader Tom Mulcair's plan for the economy.

He also invoked the memory of his famous father, former prime minister Pierre Trudeau, when making his final pitch to viewers about why Conservative jabs over his qualifications just weren't landing.

He was scrappy and aggressive, perhaps trying to play into the street-fighter image conjured earlier in the day when he invited media to watch him work out at a Toronto boxing gym.

But he took some shots of his own, specifically over the Liberal support for the Conservatives' controversial anti-terrorism law, Bill C-51, and the question of Quebec separatism.

When talk turned to Quebec and the Clarity Act, which says a vote of 50 per cent plus one vote is not enough in a sovereignty referendum, Mulcair demanded to know what threshold Trudeau would want met in a separation vote.

Trudeau started by saying he didn't question Mulcair's patriotism, but Mulcair was having none of it: "What's the number, Justin?"

Trudeau, irritated, fired back.

"My number is nine," he said. "Nine supreme court justices said one vote is not enough to break up this country, and yet that is Mr. Mulcair's position. He wants to be prime minister of this country and he's choosing to side with the separatist movement in Quebec and not with the Supreme Court of Canada."

Arguably, Trudeau had the most at stake in first leaders' debate: observers said he needed to look capable of leading the country and representing it on the world stage.

Maybe even more importantly, he had to firm up support from his party's base and "fire them up with his leadership," said Kathy Brock, a political science professor from Queen's University in Kingston, Ont.

"He's got to start looking like a leader and establish his credibility on that score."

Off the bat, Trudeau tried to frame the so-called ballot box question for voters, asking "Is Stephen Harper's plan working for you?" when asked about the economy, widely considered the top issue for voters.

He chided Harper for "continuing to invent attacks" about the Liberals, deriding as false the claims from the Tories that the Liberals would take income splitting from seniors. He chided Harper for not being able to "get it done" on the environmental file or on the economy.

"There is no public trust anymore. People don't trust this government to actually look out for our long term interest," Trudeau said.

Trudeau called Harper "disconnected," and said the Conservative leader's plan had been a failure.

Not done, Trudeau turned his sights on Mulcair, saying the NDP pledge for a $15 federal minimum wage was "false hope" because it only applied to a handful of Canadians working in federally regulated environments like banks.

A short while later, Trudeau went after Mulcair again over the NDP pledge to raise corporate taxes, saying he was pandering to those who simply disliked corporations.