Thomas Mulcair said the NDP will show Canadians they are capable of governing the country, as he laid out his plans for the party on his first day as its new leader.

The new Official Opposition leader said he won't immediately be making drastic changes to his NDP caucus, but he does intend on shaking up the Canadian political landscape.

"You're going to understand how it's important for us to be able to project confidence and competence as public administrators," Mulcair said during his first news conference Sunday.

"That's sometimes what was missing."

Mulcair, who was elected Saturday to succeed the late Jack Layton, insisted the NDP is united after the long leadership campaign but must stay focused on the job ahead.

"We're facing a government that's very tough, very well structured and we've got to do the same thing," Mulcair said.

He added New Democrats must reach out to more voters, especially young people.

"We are the only party that can stand up to Stephen Harper and form the government in 2015," he said, adding that the next election will offer voters "a very stark choice" between the Conservatives and NDP.

Warm welcome from caucus

Earlier Sunday, Mulcair opened his first caucus as leader by saying the party is prepared to face its only adversary, Prime Minister Stephen Harper.

Mulcair, who was first elected as an MP in the Montreal riding of Outremont in 2007, was given a warm welcome by his 102-member caucus at a downtown Toronto hotel.

"This is going to be a transition under the sign of continuity," he said. Interim leader Nycole Turmel joined him on stage along with Libby Davies, who will remain deputy leader. Mulcair said he will name a second deputy leader after the Easter weekend.

Speaking to reporters ahead of the party's caucus meeting, Davies, who supported veteran party strategist Brian Topp in his leadership bid, said that while New Democrats like to debate, at the end of the day they share progressive values.

"I think Mr. Mulcair is an accomplished, experienced parliamentarian," said Davies. "He's well known in the House already. Canadians know him, so I do think he brings a vigour and a force to the debate, and Mr. Harper better watch out."

Mulcair said nearly all of the people who will be part of his team are people the caucus already know.

B.C. MP Peter Julian will be caucus chair and Anne McGrath will remain chief of staff during the transition before leaving in June.

But Brad Lavigne, who was the party's national director and principal secretary to Layton and Turmel, told the CBC's Hannah Thibedeau he is leaving his positions immediately. Lavigne said he made the decision before Christmas.

"I am really excited about the party's future," Lavigne said Sunday. "The momentum started under Jack will continue under Tom. Tom is going to be a great leader and a great prime minister."

Leadership rival Peggy Nash said she expected Mulcair to reach out to the caucus.

"I think we felt unified last night during our convention, we left united," she said. "I'm really excited this morning to get to work in our caucus."

Conservatives go on attack

As Mulcair met with his MPs, political rivals were congratulating him — and taking their shots.

Speaking to CBC News on Sunday, interim Liberal Leader Bob Rae said Mulcair won with a low member vote, but conceded the next election was going to be "a very competitive race."

Rae also cast doubt on Mulcair's calls for expanding his party's support among progressives, suggesting some of the NDP's policies were outside the mainstream.

"Of course, Mr. Mulcair’s going to be reaching out, I’m reaching out, too," Rae said. "We'll just have to see how far Mr. Mulcair can reach out. Is it rhetoric?"

On Saturday, before Prime Minister Stephen Harper had a chance to congratulate the new leader of the Official Opposition, the Conservative Party had already released a statement attacking the New Democrat.

"Thomas Mulcair is an opportunist whose high-tax agenda, blind ambition and divisive personality would put Canadian families and their jobs at risk," said a statement by Conservative spokesperson Fred DeLorey.

"Mulcair has said he would bring back a risky, job-killing carbon tax which would raise the price of everything — even though Canadians overwhelmingly rejected carbon taxes," warned DeLorey.

Later, speaking to reporters in Tokyo, the Prime Minister congratulated Mulcair on his victory and said he looks forward "to interacting with him in the years to come."

New Democrats confirm the party will be releasing ads of their own on Monday in an attempt to define Mulcair's image before the Conservatives do.

Mulcair win came on fourth ballot

Mulcair was elected after four ballots and a marathon day of voting that provoked frustration among members who couldn't get on to the website to vote.

The party said the online voting system was disrupted by someone who attempted to slow it down but that the votes cast online were protected.

Mulcair beat out Topp on the final ballot following the elimination of Nathan Cullen, Nash, Paul Dewar, Niki Ashton and Martin Singh on earlier ballots.

The caucus had divided its loyalties among the candidates, but Mulcair by far had the majority of their support. As the day progressed Saturday, Mulcair picked up more support, welcoming MPs to his bleacher area on the convention floor.

Northern Ontario MP Charlie Angus immediately joined Mulcair's camp after Dewar was dropped from the ballot.

Angus told CBC News on Sunday morning that his decision was to signal that MPs should rally around Mulcair, who he described as Layton's "pitbull" in Parliament.

"He doesn't have to play the pitbull anymore," he said.

Angus also noted that Mulcair's decision to keep Davies as deputy leader should be interpreted as a nod to the party's grassroots.