The RMT and Nautilus unions are calling for action across the ports of Dover, Liverpool, Hull and Larne.
Nearly a quarter of P&O Ferries’ staff were told via a video message on Thursday that it was their “final day of employment”.
P&O said it was a “tough” decision but it would “not be a viable business” without the changes.
But unions have hit out against the dismissal, saying it marked a “dark day” in the shipping industry.
A chorus of cross-party MPs described P&O Ferries’ actions as “callous” and “disgraceful”.
The government is looking “very closely” at the action P&O Ferries has taken to see if they broke any laws after it fired its employees, planning to replace them with cheaper agency staff, it said
The RMT union labelled the move one of the “most shameful acts in the history of British industrial relations”.
Its national secretary Darren Procter told a crowd of about 250 demonstrators in Dover: “We’re going to make sure our workers get back onboard their vessels.”
P&O Ferries worker Andrew Smith said he felt “utter dismay” after working for the company for 22 years.
“It’s our lives,” he said. “It’s how our families have grown up, knowing that this is what we do, and it’s just been turned on its head within a matter of hours.”
Mark Dickinson, general secretary of the maritime trade union Nautilus, told the BBC: “It’s absolutely ripped the guts out of everybody.”
“I’ve seen some curveballs and some shocking developments over that time… but for a company to treat the legal process in such an underhand and callous way has shocked me.”
Both the RMT union and Nautilus International are seeking legal advice on the dismissal.
The transport union TSSA called on the government to “take over running vital ferry routes to safeguard trade and travel”, and “hit [P&O] where it hurts” over the “shocking events”.
Holly Cudbill, an employment lawyer from Blake Morgan, said that P&O Ferries’ actions “were almost certainly illegal”.
Although its boats sail around the world, she believes the firm’s employees are covered by UK law.
In that instance, P&O Ferries should have consulted with unions and staff about potential dismissals and notified the government that hundreds of jobs were at risk, she told the BBC.
Sacked staff said the video message had referred to a “generous severance package” being offered, but no details were given.
‘Assault on workers’ rights’
Maritime minister Robert Courts said he was “frankly angry at the way workers have been treated”. He told the House of Commons P&O Ferries’ actions were “wholly unacceptable”.
“Reports of workers being given zero notice and escorted off their ships with immediate effect while being told cheaper alternatives would take up their roles, shows the insensitive nature by which P&O approached this issue,” he said.
He added that he did not expect critical goods and services to be hit by the sudden drop in capacity, but travellers “should expect some disruption over the coming days”.
The company has said services are unable to run over the next few days.
In a letter to the prime minister, Labour’s Shadow Transport Secretary Louise Haigh described the firing as a “despicable assault on workers’ rights”.
“But British seafarers do not need meaningless platitudes – they need action,” she added, demanding that government suspends any contracts it holds with DP World – P&O Ferries’ owner.
Former transport minister Sir John Hayes also suggested the government should “recover any monies granted to P&O during the pandemic” in a bid to reverse their decision.
P&O Ferries claimed almost £15m in government grants in 2020, which included furlough payments for its employees.
The firm said on Thursday that the decision to lay-off 800 workers was “tough” but said the business would not be viable without “making swift and significant changes now”.
It said: “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.”
But the move was also condemned by the Archbishop of Canterbury and the Bishop of Dover in a joint statement, which described the sackings as “inhumane and unethical”.
“Ill-treating workers is not just business. In God’s eyes it is sin,” they said, calling for support for a consultation.
P&O Ferries is one of the UK’s leading ferry companies, carrying more than 10 million passengers a year before the pandemic and about 15% of all freight cargo in and out of the UK.
It was bought by DP World, the multi-national ports and logistics company based in Dubai in 2019. At the time of purchase, its chairman Sultan Ahmed Bin Sulayem described it as a “strong, recognisable brand”.
It paid a £270m dividend to shareholders in 2020.
However, like many transport operators, it saw demand slump in the pandemic. Just a couple of months after the dividends announcement, it said it would cut 1,100 jobs after a downturn in bookings.