St Michael's grammar school in Lurgan was closed to allow police to carry out searches following the arrests of four men in the Antrim Road area of the town last night.

The railway line between Moira and Lurgan was also closed as part of the same operation sparking congestion on the roads as many commuters were forced to take to their cars to get to work this morning.

A PSNI spokeswoman said she was unable to say how long the searches were expected to take prompting fears that commuters could face a long journey home this evening.

Information on Translink's website this morning advised passengers that due to the security operation by the PSNI there was a bus substitution between Lurgan and Moira and vice-versa.

There was also a bus substitution for passengers travelling cross-border between Belfast and Newry.

Passengers travelling cross-border from Portadown were told to board the Enterprise as normal.

The four men arrested last night were today being held at the PSNI's serious crime suite in Antrim and were helping police with their inquiries.

A PSNI spokesman said they were arrested as part of a pro-active police operation after officers stopped a car in the town yesterday. It is understood the police operation is related to combating terrorism activity.

Craigavon Borough Council's deputy mayor Mary McAlinden of the SDLP today spoke of her anger that pupils at the Cornakinnetan Road school were forced to stay at home.

"There are over 500 pupils at the school and so many of them are preparing to sit their GCSE and A-level examinations and could do without this," she said.

"This has also caused total disruption for people travelling to work. This is totally unacceptable. People don't want this.

"These people have no support in the community and I would urge anyone who has any information of any type of terrorism to go to police immediately. "