Cockroaches, locusts and crickets were released into two Byron restaurants in protest against the burger chain's alleged role in an immigration sting against its own staff.
Demonstrators released a "swarm of insects" into Byron’s Shaftesbury Avenue and Holborn diners on Friday evening - forcing the branches to close early.
Activist groups, London Black Revs and Malcolm X Movement, said they had taken "affirmative action" in response to what they called the chain's "despicable actions in the past weeks having entrapped waiters, back of house staff and chefs in collaboration with UK Border Agency".
"Many thousands of live cockroaches, locusts and crickets into these restaurants," they said in a statement on Facebook.
"We apologise to customers and staff for any irritation, however... a line must be drawn and we say, enough is enough. No one is illegal."
A journalist from Huck magazine witnessed the chaos.
Thirty-five people from Albania, Brazil, Egypt and Nepal were removed by immigration officials earlier in July.
Workers at the restaurant have accused management of organising fake meetings in order to lure the staff into the raid - something the Home Office denies.
The hashtag #BoycottByron is being used to encourage people to buy their burgers elsewhere.
Byron released a statement saying they had "co-operated fully" with the Home Office investigation.
The chain told ITV News on Saturday that it had contacted police about the insect protest.
“The safety of our customers and restaurant teams is paramount, and our priority is now to work with local police to minimise the risk of further incident.”