The Big Give Christmas Challenge has raised £10.5m for 387 charities this year.
The online match-funding scheme, which ran from last Thursday to Saturday, received 12,400 donations this year.
Funds were released at 10am on each day of the challenge and donations were matched on a first-come, first-served basis.
Donations are matched pound for pound, with 75 per cent of the match funding coming initially from the 'pledge fund', which is made up in advance by major donors and trustees of the charities taking part.
Twenty-five per cent of the match funding comes from the Charity Champion Fund, which is money contributed by individuals, foundations, corporations and families, which this year included the Waterloo Foundation, the Candis Club and the Cabinet Office, which provided £250,000 of funding to the scheme.
Once charities' pledge fund money has been used up, online donations will be matched pound for pound by the Charity Champion Fund until this runs out.
The Big Give said more than £2.5m was raised in the first five minutes of this year’s challenge, and within an hour 3,281 donations had been received and the fundraising total stood at £4.9m.
The Big Give introduced new systems last year after the 2011 challenge was marred by technical problems that caused the website to be temporarily suspended after it was left unable to cope with demand.
Dianne Parkes of the children’s brain tumour support charity Joss Searchlight said that she did not think that the challenge should involve everyone trying to donate simultaneously because it could be frustrating for donors.
She said that several of the charity’s corporate donors, who wanted to give more than £3,000, could not make their donations on the first two days. The website said the funds had been reserved, but would not allow payment to go through, she said.
The charity was able to get some donations on the third day, said Parkes.
Helen Cable, managing director of the Big Give, said: "The Christmas Challenge is a competition and this is made very clear to charities from the outset. Donations are matched on a first-come, first-served basis.
"It has been wonderful to see the fantastic outpouring of support for charities in the challenge and this has naturally resulted in the funds running out quickly. Last year, we implemented a reservations system, similar to those on ticketing websites. This was done to eradicate the problem of matched funding running out whilst a donor is making their donation, which was a problem in previous years.
"It does mean that there are times when all funds are reserved and so donors are asked to check back in a few minutes to see if they can make a matched donation."