Workers joining picket lines early today insisted they had been forced into taking action.
And some hit out at business secretary Lord Mandelson, accusing him of having a "vendetta" against union members.
At the Nine Elms mail centre in Vauxhall, south London, dozens gathered from 6am on one of the three main picket lines in the capital.
Some were brandishing banners with the message: "It's your service, it's my job."
Father-of-two John Humphries, 45, who has worked at Nine Elms since 1984, said: "None of us want to be here but we aren't concerned about what the public think, because ultimately they want the same as us.
"If changes are not made, the service will change for the worse.
"Lord Mandelson does take some blame. His problem is that he is trying to get his own back because it has been impossible to privatise us completely. He has a vendetta."
At Birmingham's main Royal Mail centre in Aston up to 100% of the workers are expected to join the picket line throughout the day, the union said.
Steve Reid, of the Birmingham district branch of the CWU, said the way workers had been treated was "appalling" and said he hoped the strike would bring about a resolution.
"People are saying we are against modernisation as a union but we are not," he said.
"Sixty thousand jobs have gone from this business in the last five years in agreement with the union.
"That's not a union against modernisation. What we want to do is get Royal Mail fit for the 21st Century, but it's got to be through agreement, not dictatorship or imposition."
Postman Mahmood Ali, a Birmingham area CWU representative, has five children to support at home in Handsworth and is fearing for his job.
The 40-year-old said: "I feel passionate about a job which was known as a job for life."
He added: "This industry desperately needs transformation.
"We believe changes and modernisation is long overdue and it should be implemented jointly with the spirit of industrial relations, whereas Royal Mail want to impose changes and so-called modernisation by diktat."
A steady stream of delivery vans were seen coming and going from the mail centre in Vauxhall despite the start of the strike.
But workers on the picket line insisted numbers would swell after the overnight staff came off shift.
Gordon Morford, 56, from Lee, south east London, said he had lost hundreds of pounds through strikes at the centre in recent weeks.
But the postal worker, who has served the Royal Mail for 35 years, said the action was worthwhile.
"We've all got mouths to feed in the build up to Christmas and, as an individual centre, we have had 15 days of strikes in recent weeks.
"But you have to reassure your families because our working conditions are unacceptable.
"There are 1,400 at this mail centre and we hope, by the end of this period, you will be able to count the number who don't join this action on one hand."
At Liverpool's main sorting office, in the city centre's Copperas Hill, about 30 workers staged an early morning picket watched over by almost as many police officers.
Mark Walsh, secretary of the local branch of the Communication Workers' Union, said he was "surprised" yesterday when last-ditch talks between the two sides ended without agreement.
He said: "I felt quite positive yesterday. The messages I was getting out of London was that we were close to a deal to call off the strikes and carry on talking.
"I think we were close to a deal but someone else has pulled the plug on it.
"Nobody wants to go out on strike. It is disappointing, especially when we were hours away from an agreement to move forward."
Mr Walsh said he hoped the strike would not last and all sides would soon "see sense".
He added: "Nobody wants to be on strike and I would hope that everyone sees sense this morning.
"The turn-out in Liverpool has been excellent and nobody has crossed our picket line. If it's the same across the country, then there needs to be some resolve by the top echelons of Royal Mail to get this sorted."
Mr Walsh said as far as he was concerned the fundamental position of the workers was to "negotiate change".
He said: "So far we have not been able to do that. We have had 63,000 job cuts in five years and we have not seen an improvement in the service and when postmen and women are going about delivering the mail they are getting complaints about the time they are turning up.
"Now that can't be good for the service, it is not good for morale. We want to negotiate the change but make sure the public are unaffected at the same time."
Asked whether the strike could last until Christmas, he said: "We don't want that, I'm certain the business doesn't want it and the public don't want it, so this has got to be resolved quite quickly.
"The way the talks ended, it looks like it is going to be a bitter dispute but we don't want a bitter dispute.
"I think the public are starting to see that we are not just going on strike for the sake of it, we have a message about protecting their service."
In Leeds, around 20 people gathered outside one of the north's biggest sorting offices which employs about 1,000 people.
A large union banner was positioned at the front of the depot, while several people held placards.
Danny McGougan, area processing rep, said he hoped about 50 or 60 people would join the picket as the day went on.
He said: "It's the intimidation of senior managers, trying to push change without negotiation. We are up for negotiation in this place.
"It is about intimidation and lack of management.
"We even have two managers standing with us today, watching what we are doing.
"Once the night shift is finished, no one will cross the picket line. We have 100 per cent support for what we are doing today."
Postman Greg Humphreys, 40, said: "I've been here 24 years and I have never known it so bad. People think it is just about pay. It is not, it is about the management and the harassment."
Pauline Bell, 53, who has worked for the Royal Mail for 15 years, said: "People think this is just about pay, but it is much more than that. It is the harassment and intimidation. There is less people to do more work."
Steve Edhouse, a delivery driver at Nine Elms, told how his mother, father and brother had all worked for the Royal Mail, with a combined service of 105 years.
But, standing at the picket line in Vauxhall, the 50-year-old, from Westminster, central London, said: "It has never been worse.
"We have built up a proud reputation over the years but now the public are being cheated and so are the workers."
As the picket line grew steadily at Nine Elms, one worker arrived with a Royal Mail bib inscribed with the words "Kant Ahfard Ate" - apparently meaning "cannot afford it" in Jamaican patois.
Mr Edhouse added: "Royal Mail mismanagement is to blame. We are being run into the ground.
"Adam Crozier, who should take most of the blame, is not even getting involved in negotiations."
He warned strikes could go on "for some time yet", saying: "We have been totally forced into this action. To implement job changes without union consultation is bound to upset people."
Around 40 striking postmen manned a picket line at the Royal Mail's main distribution centre in Peterborough, Cambridgeshire.
Local union branch chairman Andy Beeby estimated that 95 per cent of postal workers in Peterborough had voted in favour of the strike.
"The Royal Mail have done such a good job in the way they have treated us that there's been, I would guess, a 95 per cent vote in favour of strike action," he said.
"Recently we had three staff suspended for refusing to sign a document relating to work standards.
"The document didn't say what the standards were but they were then suspended for not working to them. That's the way we are treated."
He added: "We want to modernise. We know that without modernisation we will die.
"But we want to modernise through negotiation.
"What they're doing to us isn't modernisation. They're just heaping more and more work on us and imposing a pay freeze.
"This is a legitimate strike.
"In our view what they are doing is payback time from Mandelson.
"We have beaten him already twice on privatisation and he wants to smash the union."
Mr Beeby said around 10 postal workers had crossed the picket line and several vans with blacked out windows had taken recently recruited temporary staff into the distribution centre.
A steady flow of delivery vans could be seen entering and leaving the Birmingham mail centre.
About 40 workers armed with banners staged demonstrations at the front of the centre, and at the back.
The union said it expected all workers to go on strike today, with more joining the picket line later.
Steve Reid, of the Birmingham district branch of the CWU, said: "I think it's been appalling the last few months, that's the reason we are here today.
"They've imposed changes in the workplace without talking to the union. They have to talk to the union.
"We are a recognised union and until they realise that, we are going to go nowhere."
He said he was very apologetic towards the public, and added: "It's not a stance we take lightly but at the end of the day Royal Mail has forced our hand and they've backed us into a corner.
"We do sympathise with the public, we don't want to take this action, but we have no choice, unfortunately."
Postman Sajid Shaikh, who is also a union rep, said: "People are upset.
"When they heard there was going to be a deal and then Royal Mail pulled out of that, it really really upset people.
"There are public who understand why we are out here and there are public who are upset but what the public need to understand is this is about working conditions and job security."
Workers from Royal Mail's National Distribution Centre in Crick, Northamptonshire, were manning their picket in shifts outside the depot.
Standing with their CWU Northamptonshire banner and flags marking the strike action, the group were supported by passing motorists who beeped their horns and gave them the thumbs-up.
The workers said they did not want to be named for fear of a backlash.
One, who has worked for Royal Mail for 21 years, said: "A lot of lads here are married with children and have responsibilities and they've had their duties and shifts changed round to make that impossible. It's all done to upset people.
"The way people are being treated, they don't even want to give their name, because of the backlash.
"There's just two people working in there at the moment, that's how strongly people feel."
Another man added: "We're being sold down the river, slowly. It's scandalous.
"The public should know we're not standing here for the money. We haven't had a pay rise in three years.
"This isn't about pay, it's about working conditions, pensions.
"None of these blokes want to see the public without post but what else can we do, we've come as far as we can."
More than 20 workers joined picket lines at Mount Pleasant, central London.
Postman Patrick Lovelock, 48, from Romford, Essex, said he had been left £800 out of pocket by a string of strikes in London in recent months.
Joining the picket line, he said: "We have to keep telling ourselves the end goal is worth it because at the moment it is hard to maintain a living.
"We will stick with it until a deal is offered. In my 30 years of service, we have never had it so bad."
Banners and red flags fixed to gates outside the offices read: "Defend postal services", "protect our pensions" and "a decent living wage for postal workers".
Workers were seen crossing the picket line but a union spokesman insisted they were all managers.Reuse content