Police are closing William St in Lurgan due to the dangerous state of a derelict building.
The busy main route to the motorway was closed at 3.30pm and is expected to cause major traffic delays.
Local business people contacted the PSNI and NI Fire and Rescue Service on Tuesday morning after noticing the building becoming unstable.
A PSNI spokesperson said: “William St in Lurgan has been closed due to concerns a building may collapse.
“Rail passengers and motorway traffic may be affected - allow extra time.
SDLP Deputy Leader Dolores Kelly contacted Building Control this morning to reported it.
She said they carried out a survey and agreed that it was an unsafe structure.
“It’s regretable that the building has been allowed to get into such a delipated state that it causes the risk to passersby and to other businesses in the vicinity,” said the Upper Bann MLA.