The incident at Pentonville prison is one of many problems that has been revealed in a new report by the Independent Monitoring Board.
The report said an inmate had also been stabbed as he arrived for service service.
According to the watchdog: "There has been an increase in gang-related incidents during gatherings for prayer.
"On one occasion, a gang fight erupted and ministers had to run for cover. On another occasion, a prisoner was knifed as he entered."
The board said 30 staff were assaulted in just two months at the prison.
The report warned that drug trafficking, violence against staff, declining job training and "inhumane conditions" are major issues in the prison.
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