Friends and foes, both politicians and members of the public, paid tribute to Jack Layton on Monday, remembering the leader of the Official Opposition as a fighter who stuck to his principles.

Hundreds gathered outside Toronto's City Hall on Monday, writing messages of support and condolences in chalk along the front of the building.

Within hours of Layton's passing, Canadians began leaving flowers, notes and cans of Orange Crush outside his Toronto office and on Parliament hill, where the Peace Tower flag was lowered to half mast.

Layton's death sparked a massive public display of mourning, with Twitter and Facebook feeds around the country turning into a steady stream of shock and sadness after hearing of the news.

The late NDP leader died at 4:45 a.m. ET on Monday morning, surrounded by family and loved ones, after battling cancer.

Prime Minister Stephen Harper said he will always regret not getting together for a jam session with Jack Layton and said the country has lost a man of strong principles and an engaging personality.

"Jack Layton will be remembered for the force of his personality and his dedication to public life," Harper said in a statement Monday afternoon, expressing his condolences.

"We have all lost an engaging personality and a man of strong principles. Laureen and I join with people across the country in extending our deepest sympathies to Olivia and their family and all of Jack's friends and colleagues."

Harper said the two, both musicians, would often talk about getting together for a jam session, but "it always seemed we were too busy."

"I'll always regret the jam session that never was. That is a reminder, I think, that we must always make time for friends, family and loved ones while we still can."

Harper announced later Monday that Layton would be honoured with a state funeral, normally reserved for current and former governor generals and prime ministers and current cabinet ministers.

Canada has never had an Official Opposition leader die in office, so there is no precedent for state funeral plans.

Harper has said Layton will lie in state in the House of Commons on Wednesday and Thursday before Saturday's funeral at Toronto's Roy Thomson Hall, where it is hoped Canadian musicians will take part.

The news of Layton's death evoked an immediate reaction from both his opponents and supporters alike on Monday as the NDP leader was remembered as a fighter who stuck to his principles.

Anne McGrath, who served as chief of staff to Layton, said her former boss will be deeply missed.

"I feel devastated, very upset and very honoured to have been able to work with him on a daily basis for so many years," McGrath told CTV News Channel.

McGrath spent four hours with Layton on Saturday, talking about everything from federal politics to the future of the party and the goals Layton still hoped the party could accomplish.

She said there was a sense that the end could be approaching for Layton and there were certain issues he wanted to discuss before it was too late.

"He was very much full of hope and determination about the ability of the party to be able to take the gains we've made so far and to build on them and move forward," McGrath said.

"He had a lot of trust in the ability of the caucus and the members of the party and Canadians. He really believed that the message we have, the policies we have, the people we have, will continue to inspire."

Former NDP leader Alexa McDonough said Layton would want his party to continue his work and help make Canadians' lives better.

"The party will mourn, people are really, really mourning a great loss, and that will go on for a while," she said. "But what will be ringing in people's ears is Jack saying ‘Get on with the job, there are 102 New Democrats in the House of Commons today.'"

Former prime minister Jean Chretien said he admired Layton for the commitment he had to his principles and said he was "a fighter."

"He was a happy warrior and I was a happy warrior. Despite the criticism you face you get up in the morning and start again, and he was like that too," Chretien told CTV News Channel.

Former prime minister Brian Mulroney said Layton managed to pull off a "major achievement," by taking what was once essentially a Prairie party and turning it into a mainstream, national force, and eventually Canada's Official Opposition.

His other great achievement, Mulroney said, was to effectively eliminate the separatist Bloc Quebecois as a federal political party.

"He was able to seize the circumstances in the last election and transform both himself into a major leader and his party's role in history, so a major achievement there's no doubt about it," Mulroney said.

Bob Rae, the interim federal Liberal leader and a former NDP premier of Ontario, spoke to reporters and described Layton as a tireless and genuine politician who always sought solutions -- though they were sometimes unorthodox or unrealistic -- to political problems.

He said there was no difference between the public version of Jack Layton and the private version.

"What you saw in private was what you saw in public. What you saw in conversation with one or two people was what you saw in conversation with Canadians. He was that kind of person," Rae said.

He later told CTV News Channel that Layton had "remarkable" success in building the NDP from the third-ranking federal party to eventually becoming the Official Opposition, eclipsing Rae's own party in the last election.

"I think Jack's success in building the party from where it was when he took over to what it was when he died speaks for itself. He was quite remarkable," he said. "He was a tremendous retail politician: he was upbeat, he was positive, he was always looking for some constructive solutions."

Former Toronto mayor David Miller said one of his fondest memories of Layton, was when the then-city councillor fought for the rights of homeless people.

"That's a group that is not politically powerful, and he was fighting for him because he believed it was the right thing to do," Miller said.

He said Layton was ahead of his time politically, fighting for gay rights, spearheading funding for AIDS awareness and anti-smoking campaigns and on climate change.

"He got results. He wouldn't fight without finding a way to bring people on side," Miller said.

Social networking sites such as Twitter came alive early Monday morning with messages of condolence as news spread of Layton's death.

"Canada's political system has lost a great fighter," wrote Liberal MP Ralph Goodale.

"The whole country extends sympathy and love to Olivia and Jack's family."

New Democrat MP Jagmeet Singh tweeted the following: "Dear Jack Layton -- the inspiration that you are is unparalleled. Your legacy will live on -- we promise!"

Green Party Leader Elizabeth May offered "deepest sympathy to all Jack Layton's family and to all in the NDP for whom he worked so hard and long."

Even Billy Bragg, the left-leaning British alternative-rock musician posted a message mourning the loss of his political compatriot.

"A sad day, both for Canada and for those of us on the Left. Jack Layton, leader of the NDP has passed away," Bragg wrote.

CTV's Chief Political Correspondent Craig Oliver said the news of Layton's death was "infinitely sad."

He said Layton was a "great guy" who most Canadians, regardless of their political stripe, would like to share a beer or a conversation with.

"In 54 years of covering national politics I've never known anyone more committed to the country, more principled in his beliefs and perhaps the saddest thing is that he has accomplished so much and he will not be able to savour the victories he has brought this party in a very personal way," Oliver said.