The top of Abbeygate Street has been cordoned off after a loud bang and smoke came from a electricity box in the street outside Cote Brasserie.
Emergency services were called to the street after the explosion at about 11.30am.
Nadine Brailey, manager of Cote Brasserie in Bury St Edmunds, said the staff had evacuated customers and the restaurant has been closed while UK Power Networks make the box safe.
She said “We heard the bang, we evacuated everybody from the building and the fire alarm went off. We got everyone to the fire assembly point and made sure everybody was safe. The police arrived within minutes.
The box is thought to belong to either UK Power Networks or Suffolk County Council.
There was no fire as a result of the fault.