As Alex Salmond joined critics of the "fire sale", the First Minister came under pressure from Scottish Labour to explain how, in an independent Scotland, he would afford his promise to renationalise the service given the big jump in value.
Shares representing a 52.2% stake in Royal Mail were snapped up at the list price of 330p by institutional investors, who were allocated two-thirds of the stock, while ordinary members of the public - known as retail investors - were given one-third.
As dealing began, stockbrokers were overwhelmed with interest; more than 100 million shares were traded in the first hour, sending the price up to an early-morning high of more than 459p. It closed at 452.7p.
At this level, Royal Mail was valued at nearly £1.3bn more than the £3.3bn implied by the initial offer price. It later settled back to around 440p, but this was still one-third higher than what investors had paid for the stock.
Ed Miliband said: "It's a fire sale of a great institution at a knock-down price. It has been undervalued for taxpayers and undervalued for customers. It's a dogmatic privatisation by this government and they have made it even worse by undervaluing it."
Frances O'Grady, General Secretary of the TUC, likened the sale to "selling five pound notes for four quid".
At Holyrood, the First Minister said: "They've obviously under- priced the shares, because they want to be able to claim a success. Good luck to the folk who've got the shares but it's not a success for the public as a whole; it means that assets have been sold off at an under- priced level."
He added: "Of course, the problem is that people get an initial gain on their shares but as we've seen from the privatisation of essential public services, people tend to pay for it in the future."
Vince Cable, the Business Secretary, dismissed the market price debut as "froth and speculation" and insisted what really mattered was that Royal Mail was "a stable and successful company going forward".
The Prime Minister also brushed aside criticism that the Coalition had undersold a great 500-year-old British institution, insisting the sale was good for the UK economy.
Speaking on an investment mission to Northern Ireland, he said: "First of all we should celebrate the success of the privatisation of this business; it's not only good for its employees, who have become shareholders, it's not only good for the shareholders who now own this company, it's good for the company itself."
Mr Cameron explained that the company, operating in a highly competitive market, could now access capital and private-sector management skills.
He added: "As for price, let's see what happens in the days and weeks ahead. What really matters is this has got off to a very good start."
Meanwhile, Downing Street insisted the UK Government had done the right thing and that its judgment would be borne out.
But the National Audit Office, Westminster's money watchdog, said it would investigate the pricing of the float. The public applied for more than seven times the number of shares available to them while City investors, hedge funds and pension funds applied for more than 20 times the number of shares available to them.
Elsewhere, Ian Murray, the Shadow Postal Affairs Minister, called on the First Minister to explain how, given the jump in value, a Nationalist government of an independent Scotland would pay for renationalising Royal Mail. "Exactly how would they fund the purchase given the value has risen by hundreds of millions of pounds? How will they deal with the pension fund deficit? The people of Scotland have the right to know," he asked.
Mr Murray added: "Yet again, Alex Salmond has cynically used this issue to make his false case for separation; a case where he has had no discussions with either Royal Mail or the UK Government and where he can't even tell us the cost of a stamp in a separate Scotland."
In response, a spokesman for the Scottish Government said: "We're committed to the Royal Mail being in public hands, serving all Scotland's people. Recent surveys show 72% of people in Scotland oppose the privatisation. With privatisation there are real concerns that services would be reduced, especially in rural and remote areas, as well as job cuts."