Around 200 protesters gathered outside the ferry terminal compound in Hull before marching onto the site and banging on the doors of the terminal building.
The marchers, including a number of sacked workers, walked past the tied-up Pride of Hull ferry but got no response from anyone inside the P&O building.
More than 200 protesters gathered outside the Port of Liverpool.
Metro mayor of Liverpool Steve Rotheram, Greater Manchester mayor Andy Burnham and Sefton Central MP Bill Esterson joined the demonstration.
Labour MP Mr Esterson said: “I think we’ve got amazing solidarity from trade unions across the north of England and local people.
“Like everyone, I’m outraged at the appalling abuse of ordinary people by DP World.”
Protesters held flags and signs, including one which said: “Shame on you!! P&O stop the carve up.”
Lorry drivers on their way to and from the port beeped their horns in support.
Speaking at the protest at the Port of Liverpool, Greater Manchester Mayor Andy Burnham said the Government should consider nationalising P&O.
He said: “I say to the Prime Minister: you have to step in and you have to deliver on your commitment to strengthen employment law so this kind of gangster practice can’t be allowed to happen any more.”
The shadow transport secretary has said that 800 P&O workers losing their jobs is “nothing short of a national scandal”.
Louise Haigh, who joined protesters in Dover, told the PA news agency: “It’s nothing short of a national scandal that 800 P&O workers were unceremoniously dismissed on a pre-recorded Zoom call yesterday and I wanted to come straight down to Dover to stand with these workers and show my full solidarity.”
She added: “I will be taking action in Parliament next week to push the Government to take sanctions against P&O now, to push leverage on them, to force them to change course and if it’s not illegal … then we need to make sure it is immediately made so.”
She said that P&O workers “deserve more” than what happened to them.
“This is a disgraceful way for a Dubai-based conglomerate to treat British workers in this country,” she added.