Thousands of workers are already manning picket lines around Britain, including one at the main administration centre in Springburn, but further industrial action is expected to be announced today.
The new strikes will involve different groups of worker and will probably be held for three days at the end of next week.
Another strike would be disastrous for the Royal Mail, who are already trying to clear an enormous backlog of letters and parcels.
Prime Minister Gordon Brown urged Royal Mail management and postal workers to get "round the table" to solve the dispute, saying the strike was "self-defeating".
Mr Brown, who was speaking during a walkabout in the centre of Wakefield, West Yorkshire, warned: "If more and more customers leave the Royal Mail and more and more customers stop using the Royal Mail, then more jobs will be lost, so this is self-defeating."
The Royal Mail insisted that its door was still open for fresh talks to resolve the bitter dispute.
The union said the walkout was "solidly supported" as leaders stepped up their attack on Business Secretary Lord Mandelson, accusing him of telling "untruths".
Lord Mandelson urged both sides to continue talking until the deadlock was broken and raised the prospect that the conciliation service Acas could become involved.
"I don't think trading insults in this situation helps resolve the dispute. Politicising or dramatising it is useless. People need to focus on what issues are dividing them."
Pickets were joined by other union activists, waving banners and flags, with slogans including Defend Our Postal Services, and Protect Our Pensions, attracting hoots of support from passing motorists.
The atmosphere was peaceful, with pickets expressing strong support for the industrial action and hitting out at the Royal Mail and Lord Mandelson - one accusing the business secretary of having a "vendetta" against postal workers.
Up to 42,000 mail centre staff and network drivers launched a 24-hour strike today, while 78,000 delivery and collection workers will walk out tomorrow.
CWU deputy general secretary Dave Ward repeated today that he believed a "form of words" was agreed during marathon talks earlier this week, claiming the progress was "wiped out" by a last-minute intervention from Royal Mail managing director Mark Higson.
Mr Ward told GMTV that Lord Mandelson had "misrepresented" the dispute and was "frankly telling untruths".
He claimed the minister was affected by the collapse of his plan to part-privatise the Royal Mail, adding: "He is putting personal setbacks in front of settling the dispute."
Mr Higson said it was "totally outrageous" for the CWU to accuse the company of reneging on a deal, accusing the union of "walking away".
"It's now clear that the union leaders who were in the negotiations could not get the support of their own executive. It would seem that their negotiating team, led by Dave Ward, has not been able to get their national executive to approve the words that we agreed with the union on Tuesday.
"My door is open and my phone is on and I am urging them again today to meet me so that we can all sign that agreement, get the strikes stopped and give us some peace in which we can get on with delivering Christmas for our customers."
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