In scenes of high drama, the president was suddenly escorted from a briefing he was delivering about coronavirus, only to return a short while afterwards.
“Sir, we’re just going to have to step outside,” an agent could be heard saying to the president, as he approached him and spoke into his ear.
At first Mr Trump did not appear to understand what the agent meant.
“Excuse me,” he asked. “Step outside,” the agent said.
Mr Trump said “Oh”, before departing the room. He president returned some time afterwards.
“There was an actual shooting and somebody’s been taken to the hospital,” he said.
The president said the shots were fired by law enforcement personnel saying he believed the individual who was shot was armed.
“It was the suspect who was shot,” Mr Trump added.
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The shooting took place near 17th Street and Pennsylvania Avenue, just blocks from the White House.
Shortly afterwards, video posted on social media showed someone who appeared to be the shooter, being treated by medial personnel as they lay on the street.
Reports said the suspect was later transferred to a local hospital, and the District of Columbia fire department said the man suffered serious or possibly critical injuries.
Authorities were investigating whether the individual has a history of mental illness.
Mr Trump praised the work of Secret Service personnel for their work in keeping him safe.
Asked if he was shaken by the incident, Mr Trump asked reporters: “I don’t know. Do I seem rattled?”
Fox News Correspondent John Roberts told the president he had been standing outside the briefing room and heard two shots fired.
“John, you heard shots,” Mr Trump asked.
“Two shots in rapid succession,” the journalist replied.
Additional reporting by Associated Press