Cyril Banks, 91, had £9,000 stolen from him by fraudsters who pretended to be from the Met Police, officers said.
The Herts and Essex Observer set up a fundraising page after Mr Banks, from Bishop's Stortford, was interviewed on BBC Three Counties Radio.
Mr Banks said he had been "overwhelmed" by the generosity.
"It's unbelievable. I didn't realise people would offer so much," he said.
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So far, £12,034 has been raised by 455 people since the page was created on Thursday afternoon.
Hertfordshire Police said an investigation into Mr Banks' case was ongoing.
Mr Banks' friend Annie Riley said she had been "appalled" by the people who had persuaded him to withdraw £9,000 from his bank account.
"I don't know how they sleep at night," she said. "Cyril's such an amazing and giving man, he's always collecting for charity.
"At least by talking about what happened to him, we might have stopped someone else from falling victim to the scammers."
Paul Winspear, editor of the Herts and Essex Observer, said the response to the fundraising page had been "amazing".
He said: "We had a lot of comments on our Facebook page about what happened to Cyril and people were asking how they could help him, so we set a page up.
"We hit the £9,000 target within about six hours."
Mr Banks said he had even been offered £40 by a young man who overheard him telling a friend what had happened.
"It's wonderful to see such kindness and generosity," he said.
Mr Banks said he would give any money over the £9,000 he had lost to charities he supported.
"If I can help other people out of this and give back what they've given to me, I would feel I am repaying their generosity".
An online fundraising campaign to help a World War Two veteran, who was defrauded out of his life savings, has raised £12,000 in less than 24 hours.