The 30-year-old woman, named in reports as Claire Squires from Leicestershire, collapsed within yards of Buckingham Palace and the last turn into The Mall which marked the final stretch of the 26.2 mile race on Sunday.
Claire decided to run the marathon in the hope of raising £500 for the Samaritans and had set up a Just Giving page for people to donate.
As the news of her death spread across the internet, people poured onto her page to start donating. By 4.30pm on Monday Claire had posthumously raised over £50,000, and as the story gathered momentum over £5 per second began pouring in. An hour later, at 5.40pm, the total had topped £75,000 with almost 7,000 people having made a donation.
By 7.30pm donations had topped £120,000, and at 8.53pm in the evening the total raised surpassed £150,000.
Claire wrote on her Just Giving page: "Hi guys as you all know I am running the London marathon it was just going to be for fun but it's a fab opportunity to raise money for my charity the Samaritans if everyone I know could donate £5.00 that would be a great help and change lives."
Users who donated money on her page praised her efforts.
One user who gave £20 said: "My heartfelt condolences to all Claire's family & friends. I hope those like me, who did not know her, continue to give generously in Claire's memory."
Another who contributed £5 commented: "What you did and have done is an inspiration that will last, our thoughts and prayers to you and your family, you have provided a light in this world."
Catherine Johnstone, chief executive of Samaritans, said the organisation was deeply saddened at news of her death.
She said: "We are devastated following the tragic death of one of our marathon runners and are supporting the family through this very difficult time. Our thoughts are with everyone who knew her.
"We appreciate all that our marathon runners do; it is with their support, commitment and fundraising efforts that we are able to offer our vital service for people with nowhere else to turn."
In 2011 Claire successfully climbed Mount Kilimanjaro for the Royal Air Force and raised £1,430.