Royal Mail shares rose 38% to 455p in their first day of conditional dealings on the London Stock Exchange.
The hugely oversubscribed sale was priced at 330p a share, valuing the 500-year-old firm at £3.3bn. At one point, the price hit 459.75p.
Private investors received 227 shares each. In all, more than 225 million shares were traded on Friday.
Business Secretary Vince Cable told Channel Four News he could have charged a higher price for Royal Mail shares.
Asked if he could have raised the sale price when he saw the level of demand for shares, Mr Cable said: “I could have done and I could have joined the speculators and spivs.
“I’m not interested in doing that,” he said.
The shares are listed officially next Tuesday, but City institutions began conditional dealings on Friday.
Some 10 million shares were traded in the first 30 seconds when the market opened. Stockbroker Hargreaves Lansdown reported that its website was having “intermittent problems” due to the “unprecedented interest” in Royal Mail.
Tom McPhail, the firm’s head of pensions research, said it was “extremely sorry for the delays”, but was making “significant progress in clearing the backlog” and hoped to have all systems running normally again soon.
“We have experienced demand this morning which has gone off the scale,” he said.
“We now have six times the normal number of staff working on our dealing lines. We know we are not the only broker affected by such problems this morning.”
The price rise is likely to fuel debate over whether the sale of Royal Mail has been undervalued. Mr Cable has insisted that the taxpayer has not been short-changed by the privatisation.
