Distance running greats Haile Gebrselassie and Paula Radcliffe were among those stunned by the twin blasts that left at least two dead and dozens injured at the Boston Marathon on Monday.
"Horrified to hear news of bomb explosion near Boston marathon finish," Britain's Radcliffe said on Twitter before the extent of the damage was known.
"Situation looks awful, thoughts with everyone," the fastest women's marathoner in history added later. "There are some very sick people out there, who would do something like this?"
Ethiopia's Gebrselassie deplored what marathon organizers said was a bomb attack, although law enforcement officials did not immediately confirm the cause of the explosions.
"Running brings people together, but what just happened in Boston is terrible," he said on Twitter. "My thoughts are with everybody in Boston."
Race organizers said on Facebook that "two bombs" exploded near the finish line, without providing a source for the information.
"We are working with law enforcement to understand what exactly has happened," the Boston Athletic Association said.
Elite race winners Lelisa Desisa of Ethiopia and Rita Jeptoo of Kenya had already donned the traditional olive wreaths awarded the victors of the venerable race and departed when the explosions sent terrified runners, spectators and event workers fleeing.
Police did not immediately say whether the explosions were part of a terrorist attack, but marathon organizers said it was a twin bombing and media outlets reported that other unexploded devices had been found nearby.
"Very sad news about Boston, thinking of everyone whos been affected by it," Tweeted British Paralympic athlete David Weir, a six-time winner of the wheelchair division of the London Marathon and a multiple gold medallist at the Paralympic Games.
Joel Laine, head of the Paris Marathon which passed off peacefully earlier this month, told AFP he feared the explosions would have a chilling effect on the runners scheduled to compete in the London Marathon on Sunday.
"There will be without doubt a climate of suspicion for a good while surrounding these type of events. I am thinking notably of the London Marathon," Laine said. "I am thinking of the anxiety this will instill in the competitors and their families."
London Marathon organisers said they would review their security arrangements, but chief executive Nick Bitel said the race would go on.
"Our immediate thoughts are with the people there and their families. It is a very sad day for athletics and for our friends and colleagues in marathon running," Bitel said, later telling BBC Radio 5 Live: "We will not be cancelling, what we are doing, we are reviewing."
The Boston Marathon is the oldest annual marathon in the world, first organized by the Boston Athletic Association in 1897 in the wake of the first modern-day marathon competition at the 1896 Olympics.
Nowadays the event is one of six World Marathon Majors, attracting elite racers from around the world as well as thousands of recreational runners and hundreds of thousands of spectators.
Traditionally held on the third Monday of April, Massachusetts' Patriots' Day state holiday, the race features a demanding course highlighted by "Heartbreak Hill" -- a relatively modest rise that nevertheless challenges contestants because of its position late in the 26.2 mile race.
US sports agencies were quick to offer sympathy to all affected.
"The Boston Marathon is one of this country's great events and the BAA is one of this sport's finest organizations," USA Track & Field chief executive Max Siegel said.
"Runners across the country are coming together with all Americans and the people of Boston to support one another during this difficult day."
The shock wasn't limited to the athletics world.
"Prayers goes out to those involved/hurt in Boston Marathon," NBA superstar LeBron James tweeted. "WTF is wrong with people man. Just sad".