Update -- 2:51 p.m.: Bloomberg reported via Twitter that all trading will resume by 3:25 p.m., a little over three hours after trading was stopped.
Here's the earlier report from Reuters:
NEW YORK - All trading on Nasdaq, the second-biggest U.S. stock exchange, was halted on Thursday shortly after midday due to a technical problem, the exchange said.
All traffic through Nasdaq stopped at 12:14 p.m. (1614 GMT), the exchange said on its website, citing a problem distributing stock price quotes. The exchange, which lists about 3,200 companies, said it expects to resume trading later on Thursday.
Meanwhile, the New York Stock Exchange's parent, NYSE Euronext, has ceased all trading in Nasdaq-listed shares at the request of Nasdaq.
"I can't remember this happening in recent memory," said Christopher Nagy, president of consultancy firm KOR Trading and a former head of trading at TD Ameritrade.
The U.S. Securities and Exchange Commission said it was monitoring developments and was in touch with the exchanges on the matter.
Thursday's failure at Nasdaq is the latest of a flurry of high-profile glitches to hit U.S. stock market trading. The incidents, including the "flash crash" in 2010, errors related to the Facebook Inc IPO and Knight Capital's disastrous trading blowup last year, have undermined market confidence.
Earlier this week, a technical problem at Goldman Sachs resulted in a flood of erroneous orders being sent to U.S. equity options markets.
"I would not want to speculate other than to say this is huge. Everything is halted in the market," said Sal Arnuk, co-manager of trading at Themis Trading in Chatham, New Jersey,
Options trading was also halted, the exchange said.
A Nasdaq spokesman was not immediately available for comment.
(Reporting by Rodrigo Campos; Editing by Nick Zieminski, Andre Grenon and Kenneth Barry)