Some of the country's top union leaders have addressed a mass rally of striking workers in Brighton.
Royal Mail staff have walked out in a dispute over pay and 10,000 possible job cuts.
The Communication Workers Union (CWU) has accused Royal Mail CEO Simon Thompson of avoiding talks aimed at averting planned strike action.
The Union said that in recent weeks, relations between worker and employer have deteriorated, with Royal Mail serving legal notice that it was legally withdrawing from existing agreements with the CWU, as well as threatening to cut 10,000 jobs following strike action last week.
CWU General Secretary Dave Ward said: "Instead of sitting down and sorting out his problems like an adult, Simon Thompson chose to be a vanishing act instead.
"When someone like him earns £62,750 a month and can give himself six-figure bonuses, it is a disgrace that he sees it fit to disrespect our members in such a way.
"Simon can't dodge the reality that a mood of rebellion is sweeping postal workers who won't accept Thompson's hostile and bizarre behaviour.
"Postal workers go above and beyond every day, and they deserve to be treated with dignity and respect."
CWU General Secretary Dave Ward says they won’t be backing down.
A Royal Mail spokesperson said: "On Friday, 14 October, we announced losses of £219 million in the first half of the year.
"This once again demonstrates the urgent need for Royal Mail to change.
"Further strike action would materially increase our losses for the full year and may necessitate further operational restructuring and job losses.
"Four weeks have passed since we invited the CWU to enter talks with Acas to resolve the change and pay dispute. We once again urge the CWU to join us in Acas talks.
"This is the only way to reach a resolution and secure Royal Mail's future and jobs for our people.
"We apologise to our customers for the inconvenience the CWU's continued strike action will cause. We are doing all we can to minimise delays and keep people, businesses and the country connected."
In a statement Royal Mail says the CEO has an annual salary of £525,000 or around £43,750 a month.