On the eighth day of the Belfast postal workers' wildcat strike police have been called to picket lines which were blocking the entries to the Mallusk sorting office.
Meanwhile, the Londonderry Communication Workers Union refused to join the strike, repudiating the "illegal" action.
The strike has now begun to affect deliveries to outside Northern Ireland, and Royal Mail has drafted in managers from mainland Britain to act as scabs.
The BBC reported that police were called into Mallusk by the Royal Mail and the company's commercial manager David Peden said there was no other option.
"A number of people on the picket line chose to block the road into and out of the Northern Ireland Mail Centre.
"They were repeatedly asked to step aside and unfortunately didn't do so. So at that stage we had no option but to call in the PSNI," Mr Peden said.
Earlier, Londonderry Royal Mail workers refused to join their Belfast colleagues in the unofficial action.
Charlie Kelly of the Communication Workers Union in the city said "under no circumstances" would they strike as long as the action remained unofficial.
On Wednesday, striking postal staff in Belfast insisted to union representatives that they were not going back to work.
Communications Workers Union spokesman Owen Davey said after an early morning meeting that the staff's position had not changed over the illegal action.
On Wednesday, Mr Davey said: "Again we repudiated the action and asked them to go back to work, pointing out that the action was illegal.
"They were adamant that their resolve was strong and they were staying out until the Royal Mail concedes to what is a reasonable request for an independent employee and industrial relations review."
The Royal Mail reported the extent of the disruption in this notice to customers:
* No mail in north, south and west Belfast - deliveries in east of the city unaffected
* No more special delivery items accepted across NI
* No mail posted in Belfast being processed
* Items posted in NI for delivery to Britain, Irish Republic and internationally not being dispatched
* Most incoming British and international mail should be delivered
* People expecting urgent mail should contact that organisation and explore alternatives such as telephone or email
* Belfast posties' wildcat feature: