The Royal National Lifeboat Institution (RNLI) has seen a spike in donations after its chief executive defended the charity helping people trying to cross the English Channel.
But a "small number" of previous supporters have said they will be withdrawing their financial backing of the rescue service over the "polarising issue".
According to numbers seen by the PA news agency, some £200,000 was donated through the RNLI website in one 24-hour period this week.
This compares to average daily donations of about £7,000.
The charity said it has faced abuse over the past five years for its operations in the Channel.
However, it added there has been a rise in examples in the past few weeks, with one London-based crew having to call the police after being abused.
And footage of migrant rescues, including one which took place in 2019, were also released by the charity.
Following the incident, RNLI chief executive Mark Dowie praised the volunteer workers, saying the charity is "incredibly proud" of their work.
He said: "Every year, our lifeboat crews and lifeguards rescue around 30,000 people. We do not judge a casualty on what circumstances have found them in trouble.
"They go home after a shout secure in the knowledge that without their help, the person they rescued may not have been able to be reunited with their own family. That is why they do what they do."
He added: "We want to be absolutely clear that we are incredibly proud of the work our volunteer lifeboat crews do to rescue vulnerable people in distress.
"They believe in and remain focused on our core purpose, along with every member of the RNLI, to save lives at sea."
Just over a week ago, the highest number of migrants to cross the Channel into the UK in a single day was recorded.
More than 430 migrants made the dangerous journey, passing the previous daily high of 416 set in September last year, according to data compiled by PA.
PA analysis suggests more than 3,330 arrived in the country in July alone, which is more than the 2019 total.
Jayne George, the RNLI's fundraising director, said: "We are overwhelmed with the huge level of support we have received from our amazing supporters in the last couple of days.
"This was never a fundraising campaign - we simply wanted to tell the story of our crews and make it clear that our charity exists to save lives at sea."
She added: "We know that this is a polarising issue and people have strong opinions on the subject.
"Sadly, a small number of supporters have contacted us to withdraw their support."
Foreign Secretary Dominic Raab praised the RNLI, while also saying that the UK needs "to come down as hard as humanly possible" on the "wider small boats issue."
Speaking to LBC, Mr Raab said: "The RNLI do an incredible job, they have got this great heart and soul to them and they do an amazing job."
He added: "To see the threat of the criminal gangs trying to launch from France to the UK as early as possible, that is partly intelligence, it is partly technology, and secondly have the strongest criminal action including prosecution sentences against those criminal elements that feed like parasites off the human misery of those people making that journey.
"I don't think that is inconsistent, that robust approach."