The room was decorated with marching banners; the atmosphere fiery. Speakers exhorted "comrades" to stand firm, whipping them into a righteous fury against the Government. Tony Benn was given a standing ovation as he got up to speak, and his words were interrupted by shouts of "bring back Clause Four".

This was Westminster today, as postal workers gathered to protest against plans to part-privatise Royal Mail. Unions accused the Government of "betrayal" and "cowardice", threatening to strike and cut financial support to the Labour Party.

About 500 people packed into Methodist Central Hall for a rally against the Government, with more turned away at the door.

There was no mistaking the villain of the piece. Lord Mandelson, never much loved by Labour traditionalists, was condemned by speaker after speaker (there were 31 in total). The Business Secretary's face adorned dozens of fright-masks brandished by the protesters, and they left the hall to chants of "Mandelson out".

Billy Hayes, CWU General Gecretary, accused him of "straightforward political cowardice" in a rousing address.

"We're up for compromise and modernisation, but I tell you what we're not up to — the privatisation of our industry," he said. "If they privatise the Post Office, I don't care who wins the next election."

Dave Ward, deputy General Secretary, said: "We would have marched in Mandelson's constituency — unfortunately he hasn't got one. And that might be part of the problem."

He threatened that the union would begin the process of disaffiliating from the Labour Party as soon as the Bill was published later this week.

He added, to raucous cheers, that the union would not back away from industrial action. "It if means our members will have to take strike action to defeat this then we will be doing it — have no fear."

Tony Woodley, joint General Secretary of Unite, told post workers they should not let themselves be "blackmailed" by the Government over the Royal Mail's pensions deficit. This morning the Government released a letter from the chairman of pension fund trustees, warning that pensions could be halved if the part-privatisation does not go ahead. The move was branded as "disgraceful" .

The only people less popular than Lord Mandelson were bankers. They were blasted as "cretins", "spivs and speculators" by the speakers.

"If £1.3 trillion can be found to bail out the banks, let the State underwrite everyone's pension," Mark Serwotka, General Secretary of the PCS union, said.

Numerous union leaders, Labour MPs, the National Pensioners' Convention, even the Federation of Small Businesses, came to show support. The range of voices heartened the protesters. Trisha Lavelle, a CWU member carrying an "Education and Training" banner, said: "It reminds me of the poll tax protests. They [the Government] have underestimated grossly what this means to people."

Hugh Gaffney, who works for Parcelforce in Lanarkshire, had come down from Scotland for the rally. "It was a very strong rally and more so because of the number of union speakers we got," he said

He expressed firm support for a policy to "take away Labour donations that will support any privatisation".

He agreed that strikes might be necessary if the Bill becomes law. "A lot of Labour MPs are hiding when they should be standing up for the public." But he said it would only be "a very last resort. No one wants to hurt the public."

None of those in the hall saw the Royal Mail in terms of simple economics. "it's gone from being a public service to being a business and in my view it's not right," said one postal worker.

Frank Dobson, the former Cabinet minister, saw the postal service as part of the glue that united society. "The Royal Mail delivery is one of the things that keeps us together," he said.

The speeches concluded, the marchers left the hall with chants of "Royal Mail, not for sale". After making their point in Parliament Square, the protesters began to drift away. As they kept reminding us, they do have to get up very early.

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