A nurse at Royal Melbourne Hospital was taken hostage by a patient armed with a knife in an event police are describing as a "siege".

The incident occurred in a surgery ward of the Parkville hospital on Monday evening about 6.30pm when a 60-year-old Wandin North man threatened two female staff members.

According to an email sent to radio station 3AW by one hospital employee, the patient took the nurse hostage in a toilet inside the hospital for a period of time before police were called to rescue the woman.

Both normal uniform police and "special ones with big guns and wearing black clothing" arrived at the scene, the employee who wanted to remain anonymous said.