Boston residents are being told to keep “a state of heightened vigilance” after twin blasts at the city’s famous marathon killed at least three people and injured more than 140 others – bombings that are now being investigated by an FBI-led joint terrorism task force.

Chaos erupted as near-simultaneous blasts struck about 90 metres apart, the first on Boylston Street near the photo bridge that marks the finish line. Among the dead is an eight-year-old child.

In a press conference late Monday evening, Massachusetts Gov. Deval Patrick said Boston “is open and will be open tomorrow, but it will not be business as usual. There will be a heightened law enforcement presence consistent with the severity and seriousness of the ongoing investigation.”

Patrick said officials are “also asking that everyone be on a state of heightened vigilance. That is really required of everyone.”

Patrick warned that residents riding public transit should expect random checks of backpacks and parcels, “for the public’s safety.”

The head of the FBI’s Boston Joint Terrorism Task Force, which will lead the investigation, also urged residents to be on “a heightened state of vigilance,” and to call an FBI tip line with any information about the incident.

During the same news conference, Boston Police Commissioner Ed Davis confirmed that the death toll had risen to three. However, he said it would take officials until Tuesday to confirm the number of people wounded and the exact nature of their injuries.

After the first blast, paramedics and National Guard members who were working the race rushed to tear down fencing that cordoned off the spectator area from the course in order to reach and treat the injured.

"They just started bringing people in with no limbs," runner Tim Davey, of Richmond, Va., told The Associated Press. He said he and his wife, Lisa, were shielding their children’s eyes inside the medical tent.

"They just kept filling up with more and more casualties," Lisa Davey said. "Most everybody was conscious. They were very dazed."

Hospital officials across the city said at least 15 of the wounded suffered critical injuries. Canada’s Foreign Affairs Department confirmed late Monday that no Canadians were injured in the blasts.

Twenty-nine of the wounded were taken to Massachusetts General Hospital, and eight were listed in critical condition there late Monday night.

Dr. Peter Fagenholz said many suffered combined bone, soft tissue and vascular injuries to their lower extremities.

“We have performed several amputations,” Fagenholz told reporters outside the hospital.

Many of the patients had shrapnel-type injuries, he said.

Several of the patients who had surgery Monday will also require additional operations throughout the week, he said.

Alisdair Conn, chief of emergency services at the hospital, told AP earlier Monday: "This is something I've never seen in my 25 years here ... this amount of carnage in the civilian population. This is what we expect from war."

Roupen Bastajian, a 35-year-old state trooper from Smithfield, R.I., had just finished the race when he heard the blasts.

"I started running toward the blast. And there were people all over the floor," he said. "We started grabbing tourniquets and started tying legs.”

Bastajian told AP that he counted at least 25 people who had lost a limb in the explosions.

The blasts occurred at about the four-hour mark of the race, when many recreational runners are headed toward the finish line and thousands of spectators still line the route, both standing and sitting in grandstands built for the event.

The race, which includes 23,000 runners, is one of the most prestigious marathons in the world. It is held on Patriots Day, a Massachusetts holiday.

U.S. President Barack Obama vowed that, ‘We will find out who did this, and we will hold them accountable.

Obama told reporters in a short and stern address that he has directed full federal government resources to the Boston area in order to protect citizens, increase security and investigate the explosions, which rocked the area shortly before 3 p.m. local time.

“We still do not know who did this and why, and people shouldn’t jump to conclusions before we have all the facts,” Obama said. “But make no mistake, we will get to the bottom of this. We will find out who did this, and we will find out why they did this.”

Obama vowed that those responsible for the blasts will feel “the full weight of justice.”

Obama also said he told Patrick and Boston Mayor Thomas Menino that “all Americans stand with the people of Boston.”

Shortly before Obama spoke, a White House official who spoke to AP on condition of anonymity said the incident is being treated as a terrorist attack.

Police said no suspects are in custody, and authorities in Washington said no one immediately stepped forward to claim responsibility for the attack.

A senior U.S. intelligence official said two other explosive devices were also found near the marathon finish line, and were disarmed.

Investigators don’t yet know the motive for the attacks but urged people to stay indoors and avoid large groups. The Federal Aviation Administration barred low-flying aircraft from getting within 3.5 miles of the blast site.

Davis said earlier Monday that his force received “no specific intelligence that anything was going to happen” at the race.

Police said a fire at the John F. Kennedy Library may have been caused by an incendiary device, but have not confirmed whether the incidents are related.

Runners who had not finished the race were diverted down Commonwealth Avenue and into a family meeting area. Patrick said a support centre has been set up at the Park Plaza Castle for runners who were collected along the race route to meet with their families.

Near the end of the race course, officials had paid tribute to the victims of the deadly shooting in Newtown, Conn. with a mile marker flag at the 26th mile of the 26.2-mile course.

Late Monday, officials in London said that in the wake of the Boston explosions, they are reviewing the security plans for the London Marathon, which is scheduled to take place on Sunday.

London Marathon head Nick Bitel said "it is a very sad day for athletics and for our friends in marathon running."

If you're a Canadian involved in the Boston Marathon explosion and need assistance, contact @TravelGoC 1-800-387-3124 or sos@international.gc.ca.

If you are concerned for a friend that ran the Boston Marathon today, you can see their last check-in here: http://raceday.baa.org/individual.html