About 30 people were injured in the explosion that occurred in the North of London.
"Ten individuals suffered minor injuries following large fire lit as part of local Jewish community celebration in Ravensdale Road, Stamford Hill", a police statement said.
Dramatic footage shows a large crowd gathered around a bonfire before it erupted into a fireball.
However, according to the Jewish News, the bonfire had been doused in oil before being ignited and it was this that caused the explosion.
Jewish ambulance service Hatzola and the London Ambulance Service were at the scene to treat burn victims. The Jewish holiday falls on the 33rd day of the Counting of the Omer, the period between Passover and Shavuot.
Lag b'Omer also commemorates the death of Rabbi Shimon Bar Yochai, a kabbalist and mystic who was a disciple of Akiva. The custom of lighting bonfires to celebrate it has developed over the past hundreds of years and has no basis in religious law.