Seafarers from around the UK were dismissed without warning in a video message, including some based on the Cairnryan-to-Larne crossing.
The Rail, Maritime and Transport (RMT) union claims P&O Ferries brought in foreign workers from India, the Philippines and Ukraine to man the boats earning as little as £1.82 an hour.
Some British agency workers who turned up in coaches to replace the sacked staff, at Cairnryan refused to go on board when they discovered who had hired them.
The RMT will stage a boycott and demonstration at Cairnryan on Wednesday, and no crossings of the route have been made since the sackings.
The Dubai-based company justified the decision stating: “We have made a £100m loss year-on-year, which has been covered by our parent DP World. This is not sustainable. Without these changes there is no future for P&O Ferries."
Dumfries and Galloway Council leader Elaine Murray said: “We are sickened at the actions of P&O Ferries and our immediate thoughts are with the affected employees.
“We would strongly urge P&O Ferries to reconsider its course of action. If the company continues down this route, then the council will reconsider their continuing participation in a potential partnership bid for a green freeport at Cairnryan.”
Stranraer and the Rhins councillor Willie Scobie claimed the First Minister should shoulder some blame.
He said: “Nothing excuses the way P&O has behaved in sacking 800 employees with immediate effect.
"However, this should be laid at the door of Nicola Sturgeon for not doing anything to try and help the situation at Cairnryan.
"Transport Ministers knew how precarious Cairnryan is with so much competition on southerly routes on the Irish Sea and still they do nothing to the A77 and A75.”
Other politicians have written to P&O Ferries seeking an explanation,
Dumfries and Galloway MP Alister Jack said: “This is very concerning news from P&O Ferries.
"The company provides a vital link between Scotland and Northern Ireland and it is also an important employer in south west Scotland.
"I have written to P&O Ferries asking for urgent clarity on what this means for the service and for jobs.”
South Scotland list MSP Emma Harper said: “I am concerned specifically about the operations at Cairnryan. Currently, no detail has been announced about the port and I have contacted P&O Ferries to seek an update.
“P&O Ferries have a duty to their staff and I have serious concerns over how they are managing this situation.”
South Scotland list MSP Colin Smyth added: “Colin Smyth said: “This is devastating news and an utter betrayal of the Wigtownshire community by P&O Ferries.
"Workers stood by the firm during the pandemic keeping sailings going yet now P&O are walking away from them, it is simply beyond contempt and the trade unions have my solidarity in fighting these job losses.”