Three people were killed and more than 130 others were injured when a pair of blasts occurred almost simultaneously near the finish line on Monday. The Associated Press reports that a senior U.S. intelligence official has indicated that two other explosive devices were found near the finish line.
U.S. President Barack Obama appeared on television on Monday afternoon, vowing that authorities will find those responsible for the violence in Boston and said they "will feel the full weight of justice."
Among the thousands who had signed up for the race on Monday were more than 200 people from Toronto, including one woman who described seeing the wounded people being pulled into a tent for treatment.
"There are people who are really, really bloody," said Laura McLean, a runner from Toronto, who was being treated in the medical tent for dehydration when she was pulled out to make room for victims of the explosions. "They were pulling them into the medical tent."
Gerry Thuss was approaching the finish line of the Boston Marathon when the first of the two explosions occurred on Monday afternoon.
"For myself, I was actually I was on the last stretch of the finish of the marathon … and when all of a sudden, right at the finish line, there was an extremely loud explosion," Thuss told CBC News in a telephone interview.
Thuss said that explosion appeared to come from the left side of the finish line.
"And then probably a couple of seconds after, another explosion went, which was closer to me — probably 30 to 40 yards," he said.
The runner from Stratford, Ont., said that when the second blast occurred, the people on the course around him stopped moving. They were in shock.
'The ground shook beneath our feet'
Dave Cracknell, a running coach from Brampton, Ont., was in Boston to support his wife, who completed the marathon before the blasts occurred.
After taking a break following the race, the couple was heading back towards the finish line when the blasts occurred.
"There was just a huge, you know, cannon explosion sound and our feet shook," Cracknell told CBC News in a telephone interview.
"The ground shook beneath our feet and about 10 seconds later, it happened again."
Cracknell said he first thought it was possible that the grandstand had collapsed at the finish line.
"You just don't think it's going to be anything sinister," he said.
Shortly before the blasts took place, Paul Cloutier of Toronto had intended to buy a souvenir — a Boston Marathon sweatshirt — at a shop near the finish line. But he balked at the prices and left.
"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."
His 25-year-old daughter, Kiki, finished the race just minutes before the blasts. She was sitting on the ground when the explosions rattled the ground.
"A cop asked me to get up off the ground because people started running towards me," she told The Canadian Press in a telephone interview from her hotel.
"It's pretty scary, especially being Canadian. I've never been through something like this. It's scary how vulnerable you are."
'Terrible, terrible events'
Some Ontarians had posted messages on Twitter about what they had seen and what they had survived.
"Chaotic here right now. Terrible, terrible events," tweeted Canadian runner Rob Watson, of London, Ont., who finished 11th in the men's race.
"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," posted Josh Cassidy, of Oakville, Ont.
Canada's Department of Foreign Affairs has set up an emergency line — 1-800-387-3124 — to help the Canadians in Boston.
In Toronto, Mayor Rob Ford said his office has reached out to the mayor's office in Boston "to offer any assistance and support that we can give."
Premier Kathleen Wynne released a statement saying she was "sickened and dismayed by the violence that occurred in Boston today," while also offering support to those affected by what happened.
"On behalf of the people of Ontario, I send my thoughts and prayers to our friends in Boston, to the families of those killed and injured and to everyone affected by this shocking event," Wynne said.
"We are here for them and will offer our support in any way possible."