A man has been detained by police after a bomb scare shut down trains in part of Melbourne's city loop on Saturday.

Trains, which have since resumed, bypassed Melbourne Central station and people were evacuated due to a police operation in response to bomb fears.

Trains are being diverted away from the city loop. Credit:Craig Abraham

Twitter user Linton Wood said a man had climbed onto a moving train and left a bag on its roof.

Ed Brunetti said an announcement at Parliament indicated there were 'unauthorised people' in the city loop tunnel.