The Sydenham Bypass in east Belfast was closed for a time last night between Dee Street and Belfast city airport due to an "ongoing incident".
Motorists travelling towards the airport were advised to approach via Tillysburn or the Holywood Road.
A PSNI spokesman said police were "on scene".
Translink said there may have been a "possible delay" to trains as a result of a "trespass incident".
Shortly after 8.30pm, police said the incident had ended and the Belfast to Bangor road had reopened.
However, a PSNI spokeswoman said the Bangor to Belfast carriageway remained closed.
No further details were given.
