McFadden won the Boston Marathon six days ago and was back in her hotel in the city when the two bombs near the finish line exploded killing three people and injuring over 170 more.
"The race is definitely dedicated to Boston and we had huge support from London which was amazing," McFadden, celebrating her 24th birthday, said.
"I think that support definitely carried athletes through the entire race which was phenomenal and even London donating £2 for every finisher to Boston.
"Just the support that we're getting around the world means a lot especially back in Boston and to the athletes."
Undaunted by the bombings but amid increased security, the crowds were out in force enjoying the sun and the spectacle of 36,000 runners, many in fancy dress, taking on the course.
A 30-second silence was held before the start of the elite men's and mass race to honour the victims of Boston.
Competitors were provided with black ribbons to wear on their racing kit in memory of the Boston victims and London Marathon organisers were donating two pounds per finisher to the One Fund Boston set up to raise money for the victims. They hoped to raise at least £70,000.