Liberals are demanding that a Conservative member of Parliament apologize for saying that there are "extremist elements" within the Liberal party.

"We know there is an extremist element in the Liberal party generally that has been very vocal in opposing measures that are designed to combat terrorism," Ottawa Tory MP Pierre Poilievre told a radio interviewer last week.

"And it would seem that Mr. Dion has collapsed under the pressure from those groups."

Poilievre remarks come in the wake of Tuesday's vote over extending anti-terrorism measures that provide authorities with special arrest and investigative powers. The measures, which are set to expire Thursday, were introduced by the Liberals following the Sept. 11, 2001, attacks. But Liberal Leader Stéphane Dion opposes extending some of the provisions.

"All of us are looking to understand why the Liberals have had this sudden flip-flop. We're looking for an explanation of their motives," Poilievre said. "Now we know that a lot of extremist groups and people with some very hard left-wing views have advocated for a long time that these provisions should be scrapped."

Poilievre alleged that many of those people supported Dion's leadership bid.

He also said that some with the Liberal caucus want to legalize Hezbollah, and shut down the investigation into the 1985 Air India bombing.

Liberal MP Omar Alghabra called Poilievre's comments "outrageous, slanderous" and demanded an apology.

"This is the pattern that this government, this Conservative party, is following in choosing to go to the lowest level of politics that they can find to smear people just to make a political point," he said.

"Instead of focusing on the substance of the debate, they're trying to distract Canadians and smear honourable members of the House."

Liberal MP Navdeep Bains said the Liberal party is seeking legal advice about possibly suing Poilievre.

In the radio interview, Poilievre was asked whether he thought Bains was an extremist. Poilievre would only say that he doesn't comment on individuals.

Last week, Prime Minister Stephen Harper caused an uproar in the House of Commons when he appeared to suggest that the Liberals were opposed to renewing the anti-terrorism measures because they're trying to protect Bains's father-in-law from having to testify in the continuing Air India investigation.

With files from the Canadian Press