They have been inundating Legoland with abusive phone calls and messages on Twitter and Facebook calling for bosses to cancel the event.
The abuse became so upsetting for Legoland fans and staff that police asked the company to take down its Facebook account while they investigated.
The park is being hired out exclusively to the Muslim Research and Development Foundation (MRDF) and its allied organisations.
The MRDF, based in the east London borough of Tower Hamlets, is run by Haitham al-Haddad, who has been banned from speaking at a number of British universities due to his anti-Western views.
He has spoken in favour of a form of female genital mutilation, although he recognises it is illegal in Britain, and following the death of Osama bin Laden in 2011, he wrote because he was a Muslim he would “enter paradise”.
The ultra conservative British cleric, who is of Palestinian origin, is a member of the Islamic Sharia Council.
He believes music should be banned, that men and women should be segregated, and is a leader of a campaign to build a massive mega-mosque close to the Olympic Park in east London.
Although he claims his remarks on Jews were misinterpreted, he was banned from speaking at the London School of Economics in 2012 following a request from the university’s Jewish society.
Videos of his teachings and sermons are widely viewed on YouTube and there are mixed views about him in the wider British Muslim community.
Some point out that many Christian and Jewish leaders also hold “extreme” views on homosexuality, but others say he is an “outright homophobe” who should not be influencing young children.
His MRDF charity earned £260,000 in 2012 and it has a growing influence.
It attracted controversy last November when it hired out the Chessington World of Adventures theme park in Surrey for an Eid Fun Day.
The charity said because that event was so successful, it decided to hold another one at Legoland whose season officially starts five days later on March 14.
As such the park is being hired out privately and it is likely only Muslim families will attend.
Tickets are being sold for £17 and coaches will depart from the East London Mosque in Whitechapel that morning.
The organisers’ website reads: “By the Grace of Allah, on the back of our highly successful 1st Eid Fun Day at Chessington World of Adventures – we are launching our 2nd Family Fun Day at Legoland Windsor Resort – with the hope that the two events will become the standard and annual fundays for decades to come, insha’Allah.
“Family Fun Day is a family centred event where we aim to bring Halal entertainment/environments for Muslim families in the West.
“The aim is to provide a true alternative in which like minded families can enjoy safe and enjoyable time while at the same time conducive to their faith.”
However, there are fears they will encounter protesters from the far right when they arrive.
They have even mocked up an offensive poster for the demo featuring Lego “warden” welcoming two other Lego figures: one dressed like a terrorist holding a gun and another in a full Burka.
The message from the warden reads: “Muslim welcome. We hate Christians.”
Tom Bursnall, a Ukip councillor in Windsor, said there was “a great deal of resentment building up against Legoland” in the town.
He said: “There should be a peaceful demonstration. Residents are up in arms about this.
“They’re astonished Legoland is hiring itself out to man who promotes such sinister views.”
In a statement, Legoland said: “The company who have hired the park are selling tickets for the event on their website which are available to anyone, of any faith.
“This is a commercial arrangement and something we do very regularly out of season – welcoming special interest, social and business groups of all sorts.
“We certainly do not discriminate in any way.”
A message on Legoland’s Facebook page reads: “The removal of the page was requested by the Thames Valley Police as an investigation is currently underway to identify those responsible for some of our wall posts, private messages, phone calls and emails.
“Anyone posting or thinking of posting offensive and threatening messages on this page or other social media sites should be aware that the Thames Valley Police are monitoring the situation closely and we will not hesitate to take robust and swift action against anyone engaging in this type of activity.”