JustGiving has placed in "quarantine" more than 200 pages set up in response to the Manchester terrorist attack to ensure the money is not being raised fraudulently.
The pages were set up after the suicide bomb attack at an Ariana Grande concert at Manchester Arena on Monday evening in which 22 people were killed, the youngest of whom was eight years old.
Two major fundraising campaigns were launched on JustGiving soon after the attack to help victims and their families. More than 200 other individual fundraising campaigns have been created by members of the public.
A spokesman for JustGiving said it had taken extra precautions, which it refers to as "quarantine", to prevent funds being released to page owners while it ensured a plan was in place to get the money that was raised to the right people.
The combined amount raised across all the campaigns stood at more than £3.7m at midday today.
As of midday today, the Manchester Evening News fundraising appeal on JustGiving, called We Stand Together, had raised more than £1.6m from almost 47,000 people.
The We Love Manchester emergency fund, the other major fundraising initiative for those affected by the attacks, which is run by British Red Cross, had also raised more than £920,000 from more than 25,000 people by midday today.
The two fundraising appeals have joined forces to support people who were injured in or those bereaved by the attack.
Both of the Premier League football clubs in the city, Manchester City and Manchester United, have donated a combined £1m to the We Love Manchester emergency fund.
About £200,000 has been raised by other pages set up by members of the public.
Mike Adamson, chief executive of the British Red Cross, said: "The funds raised will go to people affected by the devastating attack at Manchester Arena earlier this week. We have already seen a huge outpouring of support, and we encourage the public to keep giving generously to help ensure victims and their families get the help they need."
The Charity Commission said it had taken the "unusual step" of recommending that people donate to the We Love Manchester emergency fund, which has the backing of the Lord Mayor of Manchester and the Greater Manchester Police.
The commission said it was recommending the specific fundraising campaign to prevent fraudulent campaigns taking advantage of the attack, and pointed out that the We Love Manchester appeal had the backing of numerous charities.
The commission said that other fundraising appeals launched by members of the public were also directing people to the We Love Manchester campaign.
David Holdsworth, registrar of charities for England and Wales and chief operating officer of the Charity Commission, said: "The British public are incredibly generous, especially in response to supporting those at times of suffering and need. Sadly, during such periods of especially generous giving, there is a small minority of those who seek to exploit that goodwill for fraudulent, selfish purposes.
"That is why we are encouraging people who wish to donate to give to the We Love Manchester fund, launched by the Lord Mayor of Manchester’s Charitable Appeal Trust, to ensure these funds reach those affected by the appalling attack in Manchester."
Stephen Dunmore, chief executive of the Fundraising Regulator, said: "It is vital that the public has confidence their money is going where it’s intended. That’s why we are recommending that those who wish to give donate to the We Love Manchester appeal, or to another registered charity."