SAN FRANCISCO -- Union Square, closed for nearly two hours Thursday, was reopened after officers determined a suspicious backpack found there was harmless, a police spokesman said.

At 11:50 a.m. Thursday, a police officer working an assignment near Union Square found a backpack that had "evidence that the officer determined needed to be treated with caution,'' said San Francisco Police Department spokesman Officer Gordon Shyy.

Police called in additional units, Union Square was evacuated and a one block area was shut down to vehicles and pedestrians. Union Square is located between Powell and Stockton streets just south of Post Street.

The bomb squad determined the backpack was not hazardous and crime scene investigators collected the it as evidence. A police dog also assisted with the incident, police said. The streets were reopened at about 1:30 p.m.

"The suspicious package ... has been secured," said Shyy. "There was no merit to a hazardous device."

In addition to congested streets and traffic snarls, the incident forced Nike Women's race organizers to postpone the start of noon packet pick-up for Sunday's marathon in San Francisco.

"Consumer safety is paramount to Nike. Upon discovery of a suspicious package, we alerted the local authorities and the area was immediately cleared and an inspection completed. We can confirm the package posed no danger and the area is now opened," said marathon spokeswoman Jacie Prieto-Lopez.

Police have not said if the owner of the backpack was found or what was inside it.

Staff writer Ashley McGlone contributed to this report. Contact Natalie Neysa Alund at 510-293-2469. Follow her at Twitter.com/nataliealund. Contact Kristin J. Bender at kbender@bayareanewsgroup.com. Follow her at Twitter.com/kjbender.