At least three people were killed and more than 140 were injured, including at least 17 critically, as the area around the finish line was splattered with blood, littered with shattered glass and shrouded in smoke.
By Monday evening, there were no reports of Canadians being among the dead or injured, said an official in the office of Diane Ablonczy, minister of state for consular affairs.
For some Canadians, a few seconds made all the difference between life and death.
Paul Cloutier of Toronto was cheering on his daughter and had gone into a store near the finish line to buy her a Boston Marathon sweatshirt before she finished the race.
"I was going to try and buy one in there but I thought it was too expensive so I sat down and had a coffee," he told The Canadian Press. "That's exactly where it blew up, right at the store."
It sounded like a large transformer explosion, he said. It shook the ground and filled the area with white smoke. Then there was a second blast.
"I called my wife in Toronto and said, 'Get on CNN right away. All hell just broke loose,'" said Cloutier.
His 25-year-old daughter, Kiki, had finished her race minutes before the first blast. She was still sitting on the ground past the finish line putting her sweater on when she heard a boom, the ground rattled and a plume of smoke shot up in the air.
"A cop asked me to get up off the ground because people started running towards me," she said on the phone from her hotel after the race.
"It's pretty scary, especially being Canadian. I've never been through something like this. It's scary how vulnerable you are."
Other Canucks were a little further away from the epicentre of the blasts but felt their impact nonetheless.
Jennifer Mork, of Calgary, was watching her runner husband sign his time at marathon headquarters at the Boston Fairmont Copley Plaza — a block from the finish line — when she heard two explosions in quick succession.
"It sounded like a cannon," she told The Canadian Press from the locked-down hotel. "Everyone was sort of saying, 'What was that? That's not normal'."
Mork, 33, said a huge medical tent intended for the runners became a quick makeshift hospital.
Laura McLean, a runner from Toronto, was in the medical tent being treated for dehydration when she was pulled out to make room for victims of the explosions. She says she saw people who were "really, really bloody."
Rick Avery, from Port Hope, Ont., was in the Prudential Center Mall, right near the site of the blasts, with his 14-, 11- and five-year-old children buying flowers for their mother who was in the marathon.
"All of a sudden the building shook," Avery said over the phone. An acrid smell filled the air, he said. Panic in the building caused a stampede, with hundreds of people all clamouring for the exits.
"I had to start yelling to grab onto everyone because they were literally pushing everyone over," he said.
"It was crazy. I've never seen it before in my life. We made it outside and no one knew what was going on."
For those who weren't injured, locating their loved ones became a frenzied priority.
Ottawa resident Guy Doucet was waiting for his wife to cross the finish line when he heard the first blast and saw a plume of white smoke before being rocked by a second explosion about a few seconds later.
"People started running in the opposite direction, I just followed," he said. "My main worry was seeing if my wife was OK...the last runner told my wife that she finished 38 seconds after the bombs went off."
Canadians struggled to reach family and friends at home after Boston authorities jammed cell phone service in the aftermath of the blasts. Many turned to Twitter to let loved ones know they were safe.
"Just letting everyone know I'm okay. Was across the street from the finish line explosions outside my hotel. Thank u all for the messages xo," said Josh Cassidy, of Oakville, Ont., who finished eighth in men's wheelchair race.
"Chaotic here right now. Terrible, terrible events," tweeted Canadian runner Rob Watson, of London, Ont., who finished 11th in the men's race.
When reached by The Canadian Press later in the day Watson said there was widespread panic in the athletes area after the blasts ripped through the packed streets.
"It was very unsettling. It's very terrible still, but when it first happened, it sucked really bad," he said.
"You have friends, you have family who have come to celebrate this, you don't know where they are, so everyone gets on their phones, trying to contact their loved ones, some people can't get hold of them so it's very upsetting."
For those who made it through the day safely, the relief of escaping without serious injury did little to ease the evening's tension as authorities turned out in huge numbers to secure the city core.
"There's police and ambulances all over the place and they're telling people not to go out," Toronto resident Marty Rapson said from his hotel. "People are freaking out, people are crying."
The 53-year-old, at the Boston Marathon for the second time, said many were also having trouble returning to hotels which were right by the blast site. The entire night had turned into the antithesis of a post-race experience, he said.
"You run it and it's an amazing accomplishment and then you go out and celebrate...that's a big part of it and of course no one can so it's a big pall over the whole thing."
Some 27,000 runners, including more than 2,000 Canadians, took part in the 42-kilometre race, one of the world's premier marathons.
The blasts went off about three hours after the winners crossed the finish line.
A White House official speaking on condition of anonymity because the investigation was still unfolding said the attack was being treated as an act of terrorism.
U.S. President Barack Obama vowed that those responsible will "feel the full weight of justice." A senior U.S. intelligence official said two other bombs were found near the end of the race course in what appeared to be a well-co-ordinated attack.
Alan Brookes, race director of Canada Running Series, which organizes Toronto's Scotiabank Marathon among other events said Monday's blasts have rattled the tight-knit running community.
"I certainly feel a little shaken by it, it's just such a tragic event. And as a race organizer...your heart goes out to everyone who was injured or even worse, lost their lives," he said shortly after leaving the marathon.
"The day turned from that sort of triumph and sense of joy and achievement that you get in a marathon to this terrible tragedy."
Brookes said the running community will doubtlessly be re-evaluating organizational plans for marathons going forward.
"I'm sure we'll all pull together to do everything we possibly can to revise our security and safety manuals to make sure that marathons continue to be wonderful, joyful spectacles on the world stage."
Prime Minister Stephen Harper offered his thoughts and prayers to the victims of the explosions.
"It is truly a sad day when an event as inspiring as the Boston Marathon is clouded by such senseless violence," he said in a statement. "We stand with our American neighbours in this difficult time."
Public Safety Minister Vic Toews also tweeted his concern.
"Thoughts and prayers to those injured in Boston; continuing to monitor closely as details come to light," he said.
Other political leaders joined Harper in condemning the incident and expressing their sympathy to the victims' families.
NDP Leader Tom Mulcair said the Boston Marathon is an event that brings people from all around the world together and the fact that it was targeted in this way was "particularly horrifying."
New Liberal Leader Justin Trudeau said he was "shocked and saddened" by the news.
"During what was an event worthy of celebration for thousands of people – including many Canadians – this senseless act of violence has left us reeling."
B.C. Premier Christy Clark commended first responders in Boston, and said her government would be prepared to provide any assistance that might be needed.
"Regardless of the cause, it's a dreadful thing to happen — particularly near the very spot where runners and spectators from around the globe have been celebrating athletic achievement for the past 117 years, " she said.
The Department of Foreign Affairs has set up an emergency line — 1-800-387-3124 — to offer assistance to Canadians in Boston and their families.
— with files from Colin Perkel, Lori Ewing in Toronto.