The man, arrested before dawn in central London on "suspicion of fraud by abuse of position," is in custody, City of London police said. British media named him as Kweku Adoboli, although neither UBS nor the police identified the suspect.
Two minutes before trading began on the Swiss stock exchange, the bank issued a statement that it "has discovered a loss due to unauthorised trading by a trader in its Investment Bank."
The announcement is a severe blow to the bank's retrieved reputation after the financial crisis which tarnished the standing of Swiss banking, as well as to its profit outlook and puts a question mark over the investment banking division.
UBS said: "The matter is still being investigated, but UBS's current estimate of the loss on the trades is in the range of $2 billion."
The bank added that it may also be forced to report a loss for the third quarter due to the unauthorised trade.
The news sent shares in the bank nosediving. In early afternoon trading, they were down 8.92 percent at 9.955 francs.
The bank assured clients however that their positions were not affected.
When contacted, UBS would not give further details.
In a statement to its employees, the bank said it had uncovered the fraud a day ago.
"While this news is regrettable, the fundamental strength of our company would not be affected," it said.
It called on its employees to "concentrate on the clients" saying it "counts to have their support in these uncertain times."
The bank added that it would work tightly with its risk management team and management to find out how the rogue trading could have occurred.
"It's a sad story for UBS, but also for a bank in Switzerland as a whole. Everyone is wondering how this could happen. What kinds of risk system did UBS have?" noted Claude Zehnder, chief analyst at the Zürcher Kantonalbank.
"UBS has sort of regained credibility, and now this happened," he said.
UBS has been fighting to restore its reputation after losing colossal sums in US subprime loans and the ensuing financial and economic crises of 2007-2008.
The bank had to be bailed out by the state, while clients exited en masse amid fears that it would go under.
In 2008 and 2009, the bank was also embroiled in a serious tax evasion spat with the United States.
It was forced to hand over 300 client names and pay a $780-million fine in a first case, before a second wider case in which it finally agreed to hand over data on 4,450 American clients.
The rogue trading incident could also force UBS to "think seriously about exiting investment bank as a whole or cutting it down substantially in size," noted Zehnder, pointing out that the unit was already one of the worst performers of the bank in recent years.
An analyst at another bank who declined to be named said the case was "really bad for its reputation and that of management."
He estimated that the bank would post a 500 million franc loss in the third quarter.
In 2008, French bank Societe Generale uncovered rogue trading by one of its former employees Jerome Kerviel, whose unauthorised trades lost the bank €4.9 billion ($7.1 billion at that time).
Kerviel was later sentenced to five years in jail with two years suspended, and ordered to pay back the €4.9 billion, although the French bank said it would spare the trader from reimbursing the full compensation.