UPDATE 4.50pm
The bridge has reopened, with police thanking motorists for their patience.
It is alleged the passenger of a silver Toyota Corolla hatchback became aggressive and grabbed hold of the steering wheel of the car, causing risk to the vehicle and others on the bridge.
"The driver did the right thing and stopped and emergency services came on the scene," Murray River Police District Inspector Paul Huggett said.
UPDATE 4.29pm
Murray River Police District Inspector Paul Huggett said the incident is a police operation and an ambulance was on the scene.
"Police will open the bridge as soon as they can and in the meantime we ask that motorists avoid the area," he said.
UPDATE 4.16pm
Traffic is slowly moving with the northbound lane blocked by silver Toyota Corolla hatchback. Traffic is being directed around the scene with long delays expected.
4.05pm
TRAFFIC is heavily backed up both ways both sides of the river after the Echuca-Moama bridge has been closed.
Emergency services are on the scene and motorists have been advised to avoid the river crossing until further notice.
More to come.