Detectives are searching for the organisers of a wedding attended by about 150 people in a north London school.
The windows of the state-funded Yesodey Hatorah Senior Girls' School in Egerton Road, Stamford Hill, had been covered and the gates closed to stop people seeing in when officers arrived on Thursday night.
A statement from the school, whose principal, Rabbi Avrahom Pinter, died in April after contracting coronavirus, said the hall had been leased to an outside organisation and "we had no knowledge that the wedding was taking place".
A spokesman said: "We are absolutely horrified about last night's event and condemn it in the strongest possible terms."
The Metropolitan Police originally said about 400 people had gathered inside the building for a wedding, but later said the number was closer to 150.
The school's car park is used as a drive-in Covid-19 test centre on Sundays for people who have a pre-booked appointment, while children could be seen going to class on Friday morning.
Under current lockdown rules, weddings and civil partnerships can only take place with up to six people.
Government guidance says they should only go ahead in exceptional circumstances, such as where one partner is seriously ill.
One of the organisers of the event at the Orthodox Jewish school is facing a £10,000 fine, while five others were handed £200 fixed-penalty notices.
A statement from @BoDPres Marie van der Zyl about reports of the wedding broken up by police last night.
"We unreservedly condemn this flagrant and disgraceful breach of Covid-19 regulations, which goes against Jewish teaching that preserving life is of the highest value." pic.twitter.com/NP5I6vbn4X
— Board of Deputies of British Jews (@BoardofDeputies) January 22, 2021
Detective Chief Superintendent Marcus Barnett, said: "Given the seriousness of this breach and the number of people who were in attendance, I have asked my local officers to conduct a post-incident investigation.
"This wedding broke the law and put lives in danger and I am determined to take action to identify all those responsible for organising the event and ensure they are held to account."
Hackney mayor Philip Glanville told BBC news the venue had previously been used for similar events during the pandemic.
He said in a statement: "We support the police in taking such rapid and firm action.
"We had already written to venues including this school reminding them of the regulations and are shocked that they are continuing to breach them.
"Whilst we appreciate the central role that weddings play in the life of all our communities the current regulations are there to protect everyone.
"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."
Prominent members of the Jewish community condemned the wedding, with Chief Rabbi Ephraim Mirvis calling the event "a most shameful desecration of all that we hold dear".
He said on Twitter: "At a time when we are all making such great sacrifices, it amounts to a brazen abrogation of the responsibility to protect life and such illegal behaviour is abhorred by the overwhelming majority of the Jewish community."
Marie van der Zyl, president of the Board of Deputies of British Jews, said the breach "goes against Jewish teaching that preserving life is of the highest value".
"The reckless and dangerous behaviour of those behind this event does not represent the attitude of the vast majority of British Jews, including from within the Strictly Orthodox community, who are fully aware of the terrible toll of this pandemic," she said.
"At the latest count, 740 members of the Jewish community were among the more than 90,000 UK fatalities from Covid."
A school spokesman said: "We leased our hall to an external organisation which manages all lettings and, as such, we had no knowledge that the wedding was taking place.
"We have terminated the agreement with immediate effect.
"We are investigating how this shocking breach has happened and have no plans to re-lease the premises to any third party.
"We deplore the actions of anyone in any community breaking the law and risking people's lives in this way."
It comes as police forces and the Government move to crack down further on illegal gatherings.
A No 10 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.
"We fully back the police in taking action against people who flagrantly and selfishly ignore the rules."
Home Secretary Priti Patel announced at Thursday's Downing Street press conference that fines of £800 will be handed to people caught at house parties or indoors of more than 15 people, doubling after each offence up to a maximum of £6,400 for repeat offenders.
She said on Friday: "This is deeply insulting to the vast majority of people doing the right thing by staying at home to protect the NHS and save lives.
"The Metropolitan Police acted quickly to take robust action against this unacceptable breach which has put the health of all of us at risk."
Elsewhere, the Met said the owner of a makeshift nail salon operating from a garden outbuilding in Croydon, south London, is facing a £1,000 fine, while two women found having treatments there on Monday could get £200 fixed-penalty notices.
Police have given seven people who had gathered to watch television and eat together at a home in Romford, east London, last Sunday, penalties.