An attempt to hack into the website of the Disasters and Emergency Committee (DEC) that was set up after the Asian tsunami, is being investigated.
Officers from the Metropolitan Police's Computer Crime Unit have begun an inquiry after BT blocked the attempt on New Year's Eve.
A 28-year-old man from east London was arrested and released on bail in connection with alleged offences.
Police are examining computer equipment seized during a search.
The bailed man, who was arrested under the Computer Misuse Act at an address in London's Bishopsgate on Thursday, is due to return to a police station in February.
A spokesman for the Met said the DEC website continued to be secure and the systems in place meant the attempted breach was identified and blocked very quickly.
The DEC, which estimates to raise £200m to help tsunami victims, said it was alerted by BT staff.
Chief executive Brendan Gormley reassured the public that "every penny" donated over the internet was safe and had reached the DEC.