Just minutes after the White House Correspondents’ Association dinner began on Saturday, a gunman was confronted and tackled by law enforcement officers near a security checkpoint of the Washington Hilton.

A security video posted online by President Trump showed the man running past a security checkpoint, with a swarm of law enforcement officials in pursuit.

The gunman did not make it inside the large hotel ballroom where Mr. Trump, top administration officials and hundreds of journalists had gathered for dinner around 8 p.m., officials said. A Secret Service agent was shot in his protective vest and in good condition, Mr. Trump told reporters at a White House news conference later in the evening.

One witness, the CNN news anchor Wolf Blitzer, said he was feet away from the confrontation, and that shots were fired before officers were able to subdue the man. Mr. Blitzer described how a police officer grabbed him, took him to the ground and shielded him with his body. “I just saw a big gun, and I heard the loud bangs going off,” Mr. Blitzer said.

Sam Nunberg, an aide to Mr. Trump when he announced his presidential campaign in 2015, was also nearby when the commotion broke out.

“I saw Wolf Blitzer, and then out of the corner of my eye I saw a guy running,” Mr. Nunberg said, describing the person as dressed in black, and, as best as he could tell, wearing a hood. At first, Mr. Nunberg thought the man was rushing at Mr. Blitzer. He turned again to see Mr. Blitzer on the ground, missing a shoe. Mr. Nunberg rushed into a bathroom for safety, and soon officials ferried Mr. Blitzer in as well.

Secret Service agents soon joined, counting the number of people sheltering there. The dinner guests were then told to leave the hotel, and left the bathroom in less than 15 minutes.

In the immediate aftermath of the incident, there were plenty of unanswered questions about the sudden danger at an event hosting the president, vice president and much of the federal government’s leadership.

It appeared that no guests were struck by gunfire.

In a statement, the Secret Service said the incident took place “near the main magnetometer screening area.”