British police said on Friday they had raided a wedding with 400 guests in a London school, as the government began a crackdown on illicit parties flouting national lockdown laws.

Under current lockdown rules in England, weddings can only take place in "exceptional circumstances" with up to six people present.

Officers found hundreds of people at the school, whose windows had been covered up, with Detective Chief Superintendent Marcus Barnett calling it "a completely unacceptable breach of the law".

The organiser could face a £10,000 ($13,600, 11,200-euro) fine, police said. While many of the guests fled the scene, five were fined £200.

The wedding was held at the Yesodey Hatorah school in Stamford Hill, an area with a large ultra-Orthodox Jewish community.

The school said in a statement it had leased the hall out and had "no knowledge that the wedding was taking place".

Britain is under a national lockdown introduced this month after a surge in cases and the identification of a more contagious virus variant.

But the government, police and health chiefs have expressed frustration that laws on social distancing and household mixing were still being flouted.

Vin Diwakar, NHS England regional medical director for London, on Thursday likened people risking the close-contact spread of the virus to "turning on the lights during the Blitz".

Police have previously carried out raids on illegal raves, music concerts and parties.

Interior minister Priti Patel has hiked the maximum fine for those who attend illegal house parties with more than 15 people in homes to £800.

Britain's Chief Rabbi Ephraim Mirvis tweeted that the illegal event was "a shameful desecration of all that we hold dear" and that such behaviour is "abhorred by the overwhelming majority of the Jewish community".

In November, a New York synagogue was fined $15,000 for holding an indoor wedding with thousands of guests.

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