Hundreds of people have marched in protest at ports in the “fight for justice” after almost 800 seafarers were sacked without notice earlier this month.
Demonstrations took place in Liverpool, Dover and Hull, as calls continued for a P&O Ferries boss to quit.
Crowds walked with banners and placards, chanting “P&O, shame on you” at rallies on Saturday.
The RMT union promised “more protests, more campaigning and more political pressure” in the coming days.
The marches came as a ship operated by the ferry firm remained detained for being “unfit to sail”.
The European Causeway vessel has been held at the port of Larne in Northern Ireland due to “failures on crew familiarisation, vessel documentation and crew training”, the Maritime and Coastguard Agency (MCA) said.
Transport Secretary Grant Shapps said he will not compromise the safety of P&O vessels and insisted that the company will not be able to rush training for inexperienced people.
In a move that sparked widespread outrage, the company sacked hundreds of seafarers earlier this month, and plans to replace them with agency staff on cheaper salaries.
A spokesperson for the Maritime and Coastguard Agency said: “We can confirm that the European Causeway has been detained in Larne.
“It has been detained due to failures on crew familiarisation, vessel documentation and crew training.
“The vessel will remain under detention until all these issues are resolved by P&O Ferries. Only then will it be reinspected.”
The MCA said there were no passengers or freight on board the European Causeway vessel when it was detained.
The detention of ships is based on concerns over their safety and to prevent them going to sea.
A P&O Ferries spokesman said: “European Causeway has undergone an inspection by the MCA in Larne, during which it was deemed not sufficiently ready for entry into operation.
“We shall review the findings, make any changes required and continue to work closely with the MCA to return the ship to service.”
RMT Union general secretary Mick Lynch said: “All the protests today were well attended with hundreds turning out in Liverpool, Hull and Dover to support the P&O workers in their fight for justice.
“There will be more protests, more campaigning and more political pressure this week as we ratchet up the fight and harness the public anger at the jobs massacre on our ferries.”
He said the demonstrations “show that the public is on our side regarding the P&O sackings”.
He added: “The Government need to bring in emergency legislation and prevent P&O from sailing with undertrained, super exploited, agency crews. We want to negotiate the return of all 800 sacked staff and make sure their union rates of pay and conditions are protected.”
The Trades Union Congress (TUC) tweeted a video they said showed P&O dockers in Rotterdam refusing to load freight onto a ferry set for Hull “in solidarity with the 800 seafarers illegally sacked by P&O”.
Meanwhile, Irish trade union workers gathered at Dublin Port outside the P&O terminal to send support from across the Irish Sea to P&O staff.
The rally, organised by trade union Siptu, was attended by the new Irish Labour Party leader Ivana Bacik, and organiser Jim McVeigh told the PA news agency that the rally was intended to “send solidarity”.
He added: “The best thing we could do was to have a rally at Dublin Port outside P&O so they know our concerns.”
UK Labour has written to Business Secretary Kwasi Kwarteng asking whether the Government will seek the removal of P&O Ferries’ chief executive Peter Hebblethwaite as a director under the Company Directors Disqualification Act 1986.
In a letter, the party accused the Government of “sitting on their hands” rather than taking action to hold P&O to account, adding that the “toothless response risks giving the green light to exploitation”.
Shadow transport secretary Louise Haigh said the “shameful misconduct of P&O Ferries has ruined livelihoods” as she called for the sacked workers to be reinstated and for Mr Hebblethwaite to be “barred” as a director for his role in the crisis.
Prime Minister Boris Johnson backed Mr Shapps’ call for Mr Hebblethwaite to quit.
In Larne, the local mayor of the Mid and East Antrim Council, William McCaughey, said they would support the reinstatement of the staff immediately, while Alliance East Antrim MLA Stewart Dickson welcomed the impounding of the ferry as a safety measure.
