Two lorries were reportedly blocked from entering
Police were called to a Royal Mail sorting office in Londonderry, after two delivery lorries were blocked from entering a depot.
About 30 postal workers are on the picket line at the Great James Street sorting office.
BBC journalists saw them stop the lorries from entering.
However after a short time a compromise was agreed and the lorries were left in.
The 48-hour strike, which started on Thursday, is about modernisation plans and pay and conditions.
The disruption to the postal system is to continue with the announcement of three more walkouts next week, starting next Thursday.
Charlie Kelly, spokesperson for the Communication Workers Union said staff in the city decided to take part in the national strike
"Derry was always going to join the walkout, the question was how many were going to joint it.
"I think the main stumbling block has been Lord Mandelson's input and it seems to us now the Labour party brought him in to do one thing and one thing only, smash the CWU.
"I think that has really got the staff's backs up and they are not going to stand for it."