WASHINGTON (AP) -- A uniformed Secret Service officer shot a person who drew a weapon just outside the White House Friday afternoon, a U.S. law enforcement official said Friday.

The shooting happened within view of sightseers outside the front of the building, near sidewalks crowded with families, school groups and government workers.

The White House was briefly placed on a security alert. President Barack Obama was not there - he was playing golf - but Vice President Joe Biden was in the White House complex and was secured during the lockdown, his office said.

The U.S. law enforcement official, who spoke on condition of anonymity for lack of authorization to release the information, said the person approached the Secret Service officer and drew a weapon, and then the officer opened fire.

The Secret Service later tweeted that "all Secret Service protectees are safe."

Separately, a White House official said no one associated with the White House was injured, and everyone inside the complex is safe and accounted for.

A single patient was taken to a nearby hospital in critical condition shortly after 3 p.m., said Doug Buchanan, a spokesman for D.C. Fire and EMS.

Sightseer Jenna Noelle of Austin, Texas, said she had just taken a photo in front of the White House when she noticed a man harassing an agent. Then, "as we were walking away we heard a shot fired, then some people started running away and agents had guns and were evacuating people."

"I had a panic attack," she added. "I'm doing OK now, but it was pretty freaky to be right there a second before it happened. Not really the experience we wanted," she added.

U.S. Park Police said on Twitter that the shooting happened on West Executive Drive - the street that runs perpendicular to Pennsylvania Avenue, between the White House and the Old Executive Office Building. The White House grounds were shut down to pedestrian traffic, locking staff members and reporters indoors, until the alert was suspended.