A bomb threat has forced the Victorian Parliament into lockdown.

Staff and construction workers were evacuated from the building with Victorian Police blocking entrances before midday on Thursday.

At this stage the nature of the threat is unclear with some reports of a bomb threat phoned into the Parliament.

In a statement released after midday, Victorian Police dialled down the threat and confirmed officers had inspected the scene.

"Police completed a safety check at a Spring Street building in Melbourne.

Police were called to the building about 10.45am this morning [AEST].

The building has been deemed safe."

Members of Parliament were notified before midday regarding the threat, which they say is under investigation.

Construction workers were pulled away from the site and work ceased while Police investigate the situation and a press conference planned for the day has been cancelled.