Authorities evacuated the Washington Monument and shut down traffic on nearby streets this afternoon after police received a bomb threat that later turned out to be a hoax.

Police said a man called the District's 911 center about 2:30 p.m. and threatened to blow up the monument. U.S. Park Police quickly evacuated tourists and employees from the historic structure and then shut down surrounding roads, including Independence and Constitution avenues and 15th and 17th Streets NW.

Bomb-sniffing dogs combed the building and grounds but discovered no explosives. Police reopened the monument and surrounding area at 4:15 p.m. The road closures, coupled with a heavy rain, caused widespread gridlock.

U.S. Park Police Chief Dwight E. Pettiford said authorities did not believe the threat was credible but felt they could not take chances.

"In today's times, we have to take all threats seriously . . . You can't cut any corners," Pettiford said. "We want to assure the public that we are doing the right things."

Pettiford said police were monitoring the situation in New York, where security has been stepped up on subway trains and terminals since Thursday. Mayor Michael R. Bloomberg said that the city had received a "specific threat" against the transit system.

No one was arrested for the monument scare. Other officials said that investigators were looking into the possibility that the threatening call came from someone using a pay phone at the Metro station at Tenleytown. Metro officials said that transit police planned to view surveillance tapes.

Sgt. Scott Fear, a Park Police spokesman, declined to say how frequently threats are made against the monument.

For two hours this afternoon during a major rain shower, groups of tourists were turned away from the monument grounds with shrill whistles and stern warnings from police officers encircling the obelisk. In a typical day, the monument gets about 1,700 visitors who ride to the top of the 555-foot structure in its elevator.

Police closed off 15th Street, Constitution and Independence avenues, and then widened the perimeter about an hour after the threat. Pedestrians sloshed through puddles and frantically maneuvered strollers across the Mall.

Tourists said they understood why police were being so careful.

"This is not the Washington I knew," said John Evler, who grew up in suburban Maryland and now lives in Austin, Texas.

"When I was here, it was for peace marches and protests in the '60s," he said, shaking his head. "Bomb scares. That's new. Scary world. Scary."

Dave Hunt was waiting in line to get his ticket to tour the Washington Monument when Park Service personnel turned him away. Then police officers hustled past him with bomb-sniffing dogs.

"I walked up to the top of the monument 45 years ago. Thought I could get up there again today," said Hunt, who was visiting from Kentucky and was determined to wait out the bomb threat. "I'll get back in there once it's back open. I'm not going to change my lifestyle because of what some idiot did."

Staff writer Karlyn Barker contributed to this report.