Mulcair's car passed through the screening point at the base of Parliament Hill without stopping and went through multiple stop signs as he drove up to the Hill, even though an RCMP car was following him with its lights on, CTV reported.
Mulcair, who was driving the car, reportedly did not pull over and proceeded to his parking spot behind Centre Block, and when he then spoke to the RCMP officer, he asked, "Don't you know who I am?" and suggested the officer would be in trouble for following him.
Mulcair's office said it is normal for the NDP leader to pass through the open security gate with a wave to the officer on duty.
"The officer didn't recognize him, leading to a misunderstanding," a spokesman for Mulcair said. "Once notified of the misunderstanding, he had a very respectful discussion with an officer. He then immediately went down to clear up the misunderstanding with the commanding officer."
Mulcair's office said in an email: "Mr. Mulcair apologized for the misunderstanding. No warnings or citations of any kind were issued to Mr. Mulcair. The matter was settled immediately and cordially,"
The RCMP would not explain how events unfolded.
"We're not going to comment on it," RCMP spokeswoman Lucy Shorey told CBC News. "You have Mr. Mulcair's statement. You can discuss the matter with him."
Mulcair isn't the first MP to apologize for an incident with the Parliament Hill security checkpoint.
In 2010, Conservative MP Pierre Poilievre reportedly grew impatient waiting in line to drive through the screening area, got out of his car and pushed the entrance button for the gate without waiting for his car to be inspected.
The NDP at the time of the incident heavily criticized Poilievre and said it was a "major flagrant breach."
NDP MP Charlie Angus told the House of Commons that he "thumbed his nose at the police" and asked the government to send a message that it doesn't tolerate that kind of behaviour.
Foreign Affairs Minister John Baird, who was government House leader in 2010, responded at the time that the Poilievre incident was a "trivial" matter.
Tories "insulted" by Mulcair's behaviour
The Conservatives immediately jumped on the Mulcair story in question period. Heritage Minister James Moore, standing in for Prime Minister Stephen Harper who is in Europe, said Mulcair doesn't have the temperament to be prime minister.
"If the NDP believe in accountability and responsibility their leader should show up in the House and explain himself and why he broke the law today," said Moore.
Mulcair was on Parliament Hill for meetings in the morning and left Ottawa in the afternoon for Montreal, according to the NDP.
Moore was reminded by Speaker Andrew Scheer that MPs aren't allowed to refer to the presence or absence of other members in the House of Commons. Moore did not heed the warning and in his next response to a question he again said Mulcair should be in the Commons. Scheer told Moore to "be careful" with his answers.
Almost every time the NDP asked a question about the Senate expense scandal, or other topics, Conservative ministers responded with references to Mulcair and the NDP disrespecting the RCMP and the law.
Conservative MPs also asked planted questions including one to International Cooperation Minister Julian Fantino, who is Ontario's former chief of police.
"As a former police chief I'm personally insulted by the leader of the NDP's arrogance and disrespect for those who serve on the frontlines," Fantino said, adding that Mulcair should stop opposing the government's tough-on-crime stance.