Graffiti vandals tampered with signals to stop a train in a city loop tunnel so they could graffiti it, police believe.

Police and Metro crews were called to the tunnel between Richmond and Parliament stations about 11.15pm on Saturday to investigate a badly defaced train.

Vandals stopped a Metro train in a city loop tunnel then grafittied it. Credit:Craig Abraham

It is believed the offenders stopped the train by interfering with the signals and other train equipment about half a kilometre inside the tunnel, before spraying an entire side with graffiti.

Victoria Police spokesman Leading Constable Paul Turner said the driver had to leave his cabin, walk towards the back of the train and restart it before driving to the next platform.