Police broke up a wedding party in north London, where they now say about 150 people had gathered.
Officers found the windows at the Yesodey Hatorah Senior Girls' School, in Stamford Hill, had been covered when they arrived at 21:15 GMT on Thursday.
Guests fled from the strictly Orthodox Charedi Jewish school when the police arrived. The organisers face a £10,000 fine for breaking lockdown rules.
The Met originally claimed that about 400 guests were at the gathering.
In a statement, the school said its hall had been leased out.
A spokesman for the school, whose principal Rabbi Avrahom Pinter died in April after contracting coronavirus, said "we had no knowledge that the wedding was taking place".
He added: "We are absolutely horrified about last night's event and condemn it in the strongest possible terms."
Boris Johnson supports the police for "taking action against people who flagrantly and selfishly ignore the rules", according to the prime minister's official spokesman.
The spokesman said: "Large gatherings such as that pose a health risk, not just to those who attend but those who they live with or others who they may come into contact with."
This is a most shameful desecration of all that we hold dear. At a time when we are all making such great sacrifices, it amounts to a brazen abrogation of the responsibility to protect life & such illegal behaviour is abhorred by the overwhelming majority of the Jewish community. https://t.co/pSTrVmFPu8— Chief Rabbi Mirvis (@chiefrabbi) January 22, 2021
Chief Rabbi Ephraim Mirvis, meanwhile, said the "overwhelming majority" of the Jewish community would be appalled at the event.
Rabbi Mirvis, who serves as the head of the UK's orthodox Jewish community but is not the leader of the Charedi group, called the wedding party "a most shameful desecration of all that we hold dear".
Five guests were issued with £200 fixed penalty notices, according to police, who said their inquiries had established those present at the school had gathered for a wedding.
Video shared with the Jewish Chronicle shows officers in Stamford Hill speaking with a man to explain why they are there, although he is not accused of any wrongdoing.
They are then seen arriving at the Yesodey Hatorah Senior Girls' School.
Det Ch Sup Marcus Barnett of the Met Police said: "This was a completely unacceptable breach of the law.
"People across the country are making sacrifices by cancelling or postponing weddings and other celebrations and there is no excuse for this type of behaviour.
"My officers are working tirelessly with the community and we will not hesitate to take enforcement action if that is required to keep people safe."
On Friday morning, a security guard at the school told the BBC there were more like 100 guests at the party than the much higher number given out by police.
The Met later said in a statement: "Although initial calls suggested some 400 people had attended the wedding, it is now believed that approximately 150 people were in attendance."
Stamford Hill is part of the borough of Hackney, which has a Covid-19 infection rate of 625.43 cases per 100,000 people. The England average rate is 471.31 per 100,000 people.
The mayor of Hackney, Philip Glanville, said he was "deeply disappointed" that the wedding party had taken place, despite "the number of lives that have already been lost in the Charedi community and across the borough".
He added: "Unfortunately, similar events have taken place even at this venue before and we need to be really clear how unacceptable it is.
"We will be meeting with the Rabbinate and our community partners over the coming days to see how we can prevent further incidents of this nature."
London is under an England-wide lockdown, which prevents social mixing between households.
Londoners are asked to only leave home for limited reasons such as shopping, going to work, seeking medical assistance, or avoiding domestic abuse.
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