Security guards wearing balaclavas were sent onboard P&O ferries to remove the 800 seafarers who were sacked without warning by the company.
The firm - bought by Dubai-based logistics giant DP World in 2019 - suspended all sailings from UK ports on Thursday after it claimed to be facing financial collapse.
Passengers attempting to reach Northern Ireland were turned away from the Cairnryan ferry port near Stranraer.
All P&O ferries were instead left tied up at docks across the UK.
The RMT trade union told its members to remain onboard and defy orders from company bosses to leave.
One eyewitness told the Record how a coach full of men wearing high-viz jackets was spotted outside the P&O terminal in Cairnryan on Thursday morning.
"Locals suspected they were immigration officials but in actual fact it looks like they had been hired by P&O," he said.
"I asked what they were doing there but they refused to say.
"The coach and several cars with blacked-out windows were eventually driven onto a P&O ferry despite all other traffic being turned away."
RMT general secretary Mick Lynch said: “We are receiving reports that security guards at Dover are seeking to board ships with handcuffs to remove crew so they can be replaced with cheaper labour.
“We are seeking urgent legal action and are again calling for the Government to take action to stop what is fast turning into one of the most shameful acts in the history of British industrial relations.”
Labour MP Karl Turner posted a photograph on social media showing the captain of Pride of Hull addressing workers on the ship.
Turner wrote: “They have support right across the city of Hull and the rest of the country, and are determined to stay on board for as long as it takes.”
Shadow transport secretary Louise Haigh said employers “cannot be given free rein to sack workers and replace them with agency staff”.
Conservative MP Huw Merriman, who chairs the Commons Transport Select Committee, urged the UK Government to do “everything it can to ensure that this appalling employment transaction cannot be completed”.
He added: “Concern remains as to whether this is lawful.”
Transport Secretary Grant Shapps said his officials “will be having urgent discussions with P&O about the situation, particularly of concern for their workers”.
Nicola Sturgeon said: "I’m deeply concerned at P&O announcement - due to the importance to Scotland of the Cairnryan/Larne route obviously, but also the impact on 100s of workers.
"Fire and rehire is an appalling practice and offends the basic principle of fair work. The Scottish Government will be seeking urgent talks."
Anas Sarwar, Scottish Labour leader, said: "This is a devastating decision for all the staff, their families and their communities, and an attack on Trade Unions.
"Ferry workers were on the frontline through the pandemic, keeping essential services going – and this is how they are being repaid.
"This will cost vital jobs, plunging people into unemployment in the midst of a cost of living crisis – and it will undermine pay and conditions for years to come.
"This cannot be allowed to stand – we need urgent and united action from the Scottish and UK government to put a stop to this and protect workers’ livelihoods.”
DP World was criticised for paying a £270 million dividend to shareholders at the end of April 2020 while P&O Ferries cut around 1,100 jobs as demand for travel collapsed due to the coronavirus pandemic.
P&O began operating ferries in the 1960s.
Cruise line P&O Cruises is unaffected by the developments as it is a separate business owned by Carnival UK.
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