"It was terrible, I'd never seen something like that before," Josee Hotton, a 50-year-old from St. John's told CBC Newfoundland.
Hotton, who had just completed the race before the explosions, said there was "blood everywhere, people were running everywhere."
The two bombs that exploded in the packed streets near the finish line of the world's oldest and most prestigious marathon killed at least three people and injured more than 140 in a terrifying scene of shattered glass, bloodstained pavement and severed limbs, authorities said.
The blasts went off about three hours after the winners crossed the finish line.
Boston police Commissioner Ed Davis urged people to stay calm and contact police if they have any information about the explosions. The FBI has now taken the lead in the investigation, but local police remain involved in the investigation and many officers will be on the streets of Boston tonight and tomorrow, Davis said Monday night.
Details about what caused the blasts or who was behind them were scarce Monday, but U.S. President Barack Obama said earlier in the day that those responsible will "feel the full weight of justice."
Foreign affairs officials told CBC News Monday night that so far there have been no reports of any Canadians injured in the blasts.
Steve Smith, from Bears Paw, Alta., was roughly 100 metres away from the finish line when the explosions went off.
"I just put on a burst of speed for the finish kick and I guess I was lucky that I did," Smith told CBC's Evan Solomon on Monday evening.
"Because I'd just finished crossing the finish line and they were taping the blanket to my chest when the boom went off." Smith said there was no mistaking the sound.
"One person said, 'Is that fireworks?' but we all turned and we saw the brown and white smoke. It was in a very bad spot, right by the finish line."
Smith said megaphones were telling people to keep moving forward. He said military personnel came out of a medical tent right beside the finish line to head toward the scene.
"The cops were all over the place," he said. "And then we heard sirens, and all of a sudden helicopters."
Elite runner Rob Watson, the top Canadian finisher at 11th place in the men's race, was already having lunch with his family in a nearby hotel when the explosions happened.
"It was really upsetting," Watson said. "The possibility that some people you know ... could have got hurt."
The Canadian Consulate in Boston, which is not far from the blast site, had security precautions in place, CBC's Hannah Thibedeau reported.
More than 2,000 Canadians signed up for race
The Boston Marathon website says more than 2,000 Canadians were signed up to run the race. Nearly 27,000 people in total were signed up for the roughly 42-kilometre race, though fewer are believed have actually participated.
Carolyn Leckie of Ottawa crossed the finish line just six minutes before the explosion. "In six minutes, I probably only made it half-a-block, a block away from the finish line," she said.
"And then we all heard a tremendous explosion, we turned around and we saw just a huge, huge cloud of smoke." Leckie said by the time the second explosion happened, people were "kicking into gear" and volunteers were trying to sweep runners out of the area."
She said as they moved forward, emergency vehicles were coming up the roads, which were crowded with runners.
People were on the edge of panicking, Leckie said, but she credited volunteers who helped keep people calm.
Francine Comeau of Beaver Bank, N.S., told CBC News the explosions happened behind her, not long after she crossed the finish line.
"Everybody was begging to borrow everybody else's phone and I finally found a phone that worked, because you couldn't get a connection," she said.
"When I found a phone that worked I sent [my family] a text but within a minute I saw my family arrive, and of course everyone is crying when you seen each other because you realize what could have happened," she said.
Lesley Holmes of Ottawa finished the marathon and proudly collected her medal, but the moment of joy turned to horror.
“That seemed so important, to have such a great race, and now, after the events that unfolded today, that means absolutely nothing.”
Google has stepped in to try and help people connect after the explosions. The internet giant launched a site called "Google Person Finder" that allows people to enter the name of a person they are looking for or information about someone who is in the area.
'Fear and shock'
Kirsten Fleming, executive director of the Scotiabank Calgary Marathon, said she saw dazed runners walking through the streets
"People were quite upset and crying," she said, saying some had bandages on their heads or blood on their shirts. Fleming, who wasn't running the race, was at a nearby restaurant when the blast happened.
"The mood just went to fear and shock," Fleming said.
Davis said police had locked down the area around the finish line, and that an area as large as 15 blocks could be locked down for 24 hours or longer.
Hours after the blast, the streets were "getting very quiet" said Canadian runner Lesley Holmes, who noted that the streets of Boston are normally full of people after the race.
"It's pretty sombre in here," Canadian runner Michael Doyle said Monday night from an area near the scene.
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.
The Department of Foreign Affairs has set up an emergency line — 1-800-387-3124 — to offer assistance to Canadians.