Two bombs ripped through the crowd at the finish line of the world's oldest annual marathon, killing three people, maiming others and injuring more than 100 in what a White House official said would be treated as an "act of terror".
It was the worst bombing on U.S. soil since security was tightened after the attacks of Sept. 11, 2001.
"This ghastly and cowardly attack strikes at the very core of the freest of human activities," International Association of Athletics Federations' president Diack said in a statement.
"Whether as a fun runner or elite competitor, road running exemplifies the honesty of basic human movement, strength and resilience.".
More than 20,000 runners were taking part in the race in Boston with thousands more lining the city streets.
"We stand firm with the race organisers of the Boston Marathon...and the people of Boston at this time of tragedy and condemn this mindless attack," Diack added.
"What makes this incident so vile and its planning so incomprehensible is that marathons the world over are about selfless acts of human generosity."
International Olympic Committee President Jacques Rogge said the blasts would trigger a common resolve among those left behind.
The Olympics have had their share of tragedy with the 1972 Munich Games killings of 11 Israelis by Palestinian gunmen and the deadly 1996 Atlanta Olympics bomb blast.
"My thoughts and the those of all the Olympic Movement are with the victims, their friends and their families today," Rogge said.
"A marathon is a special almost magical event that unites different communities with one shared goal, and I am sure that this incident will serve to bring together all those who took part and all those who support such an event with a common resolve."
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