Harper says it's a sad day when such an inspiring event is clouded by "such senseless violence."
He says Canadians are standing with their American neighbours.
New Democrat Leader Tom Mulcair is also offering his condolences to the victims, their families and all Americans.
Canadians in Boston requiring emergency assistance are being urged to contact the consulate general at (617) 247-5100.
Friends and relatives seeking information on Canadian citizens believed to be in the area are asked to contact 1-800-387-3124 or firstname.lastname@example.org.