Nearly 700,000 small investors saw their stakes increase by around £280 as the price closed up more than a third at 455p, while around 150,000 postal workers could be sitting on stakes worth more than £3,000.
But amid the flotation fever in the City, ministers were facing accusations that financiers have been allowed to profit from public assets being sold off cheaply.
Shares representing a 52.2% stake in Royal Mail had been snapped up at a price of 330p by institutional investors, who were allocated two-thirds of the stock, and ordinary members of the public - known as retail investors - given a third.
The price soared as conditional dealing began, overwhelming stockbrokers with interest and crashing the website of one major brokerage, Hargreaves Lansdown.
More than 100 million shares were traded in the first hour, sending the price up to a high of almost 460p.
Retail investors who cashed in at that level netted a paper profit of around £290, based on their allocation of 227 shares each.
Business Secretary Vince Cable dismissed claims it had been undervalued, telling BBC Radio 4's Today programme the sharp price rise was no more than "froth and speculation" and insisting the sale was a good deal. He said long-term, stable investors had picked up the bulk of shares.
And Prime Minister David Cameron also brushed off concerns when he was asked about the privatisation during a visit to Bombardier aircraft factory in Belfast.
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.
"Royal Mail is in a very competitive market, it can now access capital and access private sector management techniques."
Mr Cameron said the privatisation was good for all the economy. 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."
But Labour leader 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.
"Its a dogmatic privatisation by this government and they have made it even worse by undervaluing it.
"The Government are doing it for ideology and it's not in the best interests of this country."
Many will not be able to sell until full trading in the shares begins next Tuesday but those who bought through brokers were already able to cash in, with bundles of 227 shares being sold off throughout the morning.
The actual value of around 150,000 Royal Mail employees' 10% stake in the company will be priced on Tuesday, although based on today's close this would work out at more than £3,000 per employee. However, staff cannot sell for three years. A further 15,000 staff took part in the offer to buy shares.
Bristol-based Hargreaves Lansdown apologised after "unprecedented demand" crashed its website and left phone lines jammed - leaving investors unable to trade.
Its chief executive Ian Gorham said the brokerage had expected and prepared for substantial demand, but volumes had "gone off any conventional scale".
He insisted other stockbrokers had experienced similar problems and said: " We have six times the normal number of dealing staff working today, and continue to work hard to deal with the demand.
"We will keep working flat out until our normal fluent service is restored. We'd like to apologise to our clients for any issues they have experienced this morning."
Richard Hunter, head of equities at the brokerage, said: "There's been a fair amount of selling interest. Some people have decided to bank their profits. Having said that, the stability of the price suggests those sales are being mopped up as well."
Joe Rundle, head of trading at ETX Capital, said it was a "dazzling stock market debut". But he said concerns such as the threat of industrial action, a lack of adequate capital and unclear growth strategy, could weigh down the price in future.
A demonstration against the sell-off was held outside the London Stock Exchange today, with protesters dressed as robbers.
Billy Hayes, general secretary of the Communication Workers Union, described it as "a tragedy" and told Today that despite the free shares for employees, a ballot next week was likely to back industrial action.
"Vince Cable, one of the cleverest men in British politics, has made one of the stupidest decisions he is ever likely to make as a politician," he said.
Mr Hayes said there would be no celebrations over the share surge at delivery offices around the country, amid fears that privatisation would lead to reduced services and higher prices for the consumer.
TUC general secretary Frances O'Grady said: "Privatising Royal Mail has become little different from selling five pound notes for four quid. No-one can be blamed for wanting a share of that, but let's not forget that this has taken something that belonged to all of us and given a large slice away for free to those who could afford an entry ticket.
"And everyone knows that in the long run the postal service will get worse, just as other privatised industries have ended up abusing markets and ripping off consumers."
Taxpayers have been left with a 37.8% stake in the company, though this could be reduced to 30% depending on a so-called "over-allotment" option which is dependent on price performance following the flotation.
Some retail investors were left disappointed as the share allocation was announced yesterday, as those who applied for more than £10,000 worth got nothing.
The surge in its shares could see Royal Mail slot straight into the top tier of UK shares, the FTSE 100 Index, rather than the FTSE 250 second tier.
That could stoke more demand for its shares, as FTSE 100 tracker funds are forced to hold the stock, but any promotion will not happen until the next FTSE reshuffle in December.
© 2013 Press Association