Technology problems plagued Nasdaq's computer system Friday afternoon,  forcing the No. 2 U.S. stock market to halt trading for the second straight day and extend the trading day.

The problems with two key trading systems forced the Nasdaq to extend its regular trading session by an hour to 5 p.m. EDT and cancel the late trading session it regularly holds.

Officials of the computerized stock market said an error by an MCI Worldcom technician led to the problems. 

``This was unrelated to system capacity'' or other problems at the Nasdaq, spokesman Scott Peterson said. ``Human beings make mistakes.'' 

It was the second straight day of trading problems for the Nasdaq, but Peterson stressed that Thursday's glitches were unrelated to Friday's troubles.

On Thursday, Nasdaq was forced to shut down its systems after traders reported problems entering price quotes shortly after trading in Microsoft Corp. resumed following a halt of more than three hours. Nasdaq shut down its SelectNet and SOES stock trading systems at 3:02 p.m. EDT. The quote systems were fixed and trading was restarted 18 minutes later. 

Nasdaq, which trades around 2 billion shares a day on average, recently has encountered problems with its trading systems, including a SelectNet failure earlier in June. The system also went down for about 15 minutes in March, and a handful of erroneous trades entered into the system in February caused the Nasdaq Composite Index to falsely spike up 14 percent. 

Nasdaq's new trading system, nicknamed SuperSOES, is scheduled to be implemented on July 8. SuperSOES, which Nasdaq has said has performed well in testing, enables traders to process more trades automatically. 

Technology problems also have plagued the New York Stock Exchange, Nasdaq's main rival. A problem with new trading software caused the Big Board to halt trading on its entire floor for nearly 90 minutes on June 8.

 -- Reuters and the Associated Press contributed to this report.