POLICE continue to investigate an explosion at Barwon Prison, home to some of the state’s worst criminals, this afternoon.

The explosion happened when objects were moved during a search of an unoccupied cell, the Herald Sun understands.

It is believed no parcel was sent to the jail and the incident is not being viewed as malicious.

Prisoners and prison workers escaped injury after the blast at Victoria’s highest security jail.

The incident happened just before 1pm as prison officers conducted a routine intelligence search.

A Corrections Victoria spokesman said the prison, near Geelong, had been locked down following the “very small explosion”.

“There was no physical damage and no injuries, but police, fire and ambulance services have been notified and are attending,” he said.

“It’s being treated as an isolated incident, and the prison has been locked down while authorities are on site.”

He said Corrections Victoria would investigate the incident alongside Victoria Police.

It has not been confirmed in which part of the prison the incident happened.

Police and paramedics rushed to the site.

media_camera A fire officer and jail staffer outside HM Prison Barwon today. Picture: Peter Ristevski

media_camera Emergency vehicles outside Barwon. Picture: Peter Ristevski

Police would not confirm if the bomb squad was at the prison.

Notable inmates at Barwon include Hoddle St killer Julian Knight, multiple murderer Peter Dupas, serial killer Paul Denyer and underworld figure Tony Mokbel.

Carl Williams was beaten to death at the prison in 2010.

In another isolated incident, double murderer Barry Robert Quinn was murdered inside Pentridge Prison 30 years ago today.

READ MORE: The most infamous (not-so-subtle) jail killings in Victoria

There has been a small explosion at Barwon Prison. No serious injuries. Area has been evacuated and emergency services are attending. — Victoria Police (@VictoriaPolice) July 4, 2014