Trains to Aberdeen from both Edinburgh and Glasgow have been cancelled due to a “police incident” on the line.
Bus replacement services have been introduced to cover routes between Waverley and Queen Street and the Granite City due to the issue between Arbroath and Montrose.
A Police Scotland spokesperson confirmed they were supporting British Transport Police in the operation.
A spokesperson said the incident surrounded concern for a person.
Passengers are being urged to check their journeys before setting off.
ScotRail said the line would likely be closed for some time “due to the complexity of the incident”.
Rail tickets are now being accepted on local bus services including CityLink between Dundee and Aberdeen and the Stagecoach East service between Dundee and Arbroath.
A Police Scotland spokesperson said: “We were made aware of concern for a person at Lunan Bay near Arbroath around 1.20pm on Tuesday, June 5.
“Emergency services are currently at the scene.”
The British Transport Police have been contacted for comment.