Opposition Leader Tom Mulcair says the NDP will fight the Conservative government's new anti-terrorism bill when it goes before the House on Wednesday and pushed for the Liberals to do the same.

After his party's weekly caucus meeting, Mulcair said the real threat of terrorism requires responsible measures, not the "dangerous, vague, ineffective" Bill C-51.

"Mr. Harper and the Conservatives have intimidated the Liberals into supporting this deeply-flawed legislation. We in the NDP are going to fight it," Mulcair told reporters. "The truth is we cannot protect our freedoms by sacrificing them."

Bill C-51, which would give more powers to law enforcement and intelligence agencies, is up for second reading and debate today.

The bill, introduced by Prime Minister Stephen Harper in January, would allow the Canadian Security Intelligence Service (CSIS) to "disrupt" or "counter-message" terror activity inside and outside of Canada and make "promotion" or "intentional advocacy" of a terrorism illegal instead of the current laws against counselling or actively encouraging a specific terrorism offence.

It would also lower the threshold for arrest to when law enforcement agencies think a terrorist act "will be carried out" to when they believe one "may be carried out."

Prime Minister Stephen Harper answers a question during question period in the House of Commons in Ottawa on Wednesday, Feb. 18, 2015. (Sean Kilpatrick/Canadian Press)

Harper told MPs in question period Wednesday the bill does not cover "lawful advocacy, protest, dissent or artistic expression" when asked by Mulcair about activities that would become illegal under C-51.

"This is, in fact, the attempt of the NDP to always say that anything that is in defence of our security somehow undermines our freedoms. That is simply not true," Harper said.

"All we are seeing is as the NDP's positions on this issue become more and more irrelevant, more and more unconnected to Canada's real concerns, their statements on the issue become more and more extreme."

In response to another question from Mulcair, Harper pointed to the 9/11 attacks as an example of what interfering with a country's economic stability means in the context of the bill, a part of the bill questioned by Mulcair earlier in the week.

Government trying to 'posture and position': Mulcair

Liberal Leader Justin Trudeau has already said his party will support the bill, although he says he will push for greater oversight of CSIS. But Trudeau says the bill's failings aren't enough for the party to vote against C-51.

Green Party Leader Elizabeth May is vehemently opposed to the bill and says she'll present amendments, but that the best way to deal with it would be to scrap it.

The proposal has been criticized for its potential effects on free speech, privacy and the ability to protest, but former CSIS assistant director of intelligence Ray Boisvert told CBC News the day the bill was announced that it's "very, very necessary" to give more power to disrupt when there is an immediate threat to life.

Mulcair said the fact the bill was presented at a "campaign-style event" in Richmond Hill instead of in Parliament shows the bill is politically motivated.

"What they're trying to do is posture and position," he said.

Mulcair said the NDP will do what it can to delay the bill's passage and get it before a committee, where the party can introduce amendments that would involve more input from experts and elected officials and bring Canada's approach closer to that of the U.S. and U.K.

"We've done this in the past, we've used everything that was at our disposal as parliamentarians, everything that we have in the books, to make sure that on the important electoral bill (C-23) that everybody said the Conservatives with their majority were just going to push through as is, we got a lot of amendments through," he said.

"The Liberals simply said 'well we're not going to fight this, we'll change it once we're elected,' well that's not the way to stand up to Stephen Harper's Conservatives."

Liberal Leader Justin Trudeau speaks during a news conference on Parliament Hill in Ottawa on Wednesday, February 18, 2015. (Sean Kilpatrick/Canadian Press)

After the Liberal caucus meeting, Trudeau said strengthening the no-fly list, improving coordination between national security agencies and loosening rules around preventative arrest are "concrete" measures in C-51 that increase the safety of Canadians.

"We have heard concerns from Canadians and we share them, and that's why we are unequivocal that this bill needs oversight and a review process, which we have committed to bringing in," he added.

"We will pressure and continue to pressure the government and encourage the government to adopt these reasonable oversight and review measures when [the bill] comes forward to committee, but if they choose not to bring them in we will campaign on them and we will bring them in for Canadians after the next election."

MacKay on terrorism definition: 'look it up'

Justice Minister Peter MacKay gave a curt answer when asked by CBC News' Chris Hall about the definition of terrorism in bill C-51.

"Look it up," he said as he left the Conservative caucus meeting.

MacKay's office later defended the bill in a statement responding to the NDP decision to oppose it.

"When developing any legislation or legal amendment, the Department of Justice takes every consideration in safeguarding constitutionally protected rights and freedoms," it said.

"This particular legislation also includes judicial oversight and discretion; the requirement for Attorney General consent; annual reporting requirements on recognizance with conditions; and a sunset clause requiring Parliament to consider the renewal of the recognizance with conditions."