A man who said he was investigating a conspiracy theory about Hillary Clinton running a child sex ring from a pizza restaurant in Washington DC has been detained after firing an assault rifle inside the shop.

Metropolitan Police Department spokeswoman Aquita Brown said police received a call on Sunday afternoon local time about a man with a weapon in Connecticut Avenue, an affluent neighbourhood of the US capital.

Edgar Welch, 28 of Salisbury, North Carolina, walked into the Comet Ping Pong restaurant and pointed a gun in the direction of an employee, according to the Washington Post.

The worker managed to flee and alert police. Welch then fired the gun into the ground.

Police arrested Welch without incident and recovered an assault rifle, Ms Brown said. He has been charged with assault with a dangerous weapon.

Two firearms were recovered inside the restaurant and an additional weapon was recovered from the suspect's vehicle, police said.

The Comet Ping Pong is in a neighbourhood of well-tended private homes and apartment buildings in leafy streets that lead to a mix of shops, restaurants and the Politics and Prose bookstore.

The restaurant gained notoriety during the presidential campaign after fake news stories stated that Democrat Hillary Clinton and her campaign chief ran a child sex ring out of the restaurant, news organisations reported.

Welch told police he had come to the restaurant to "self-investigate" the fictitious online conspiracy theory that spread during Mrs Clinton's run for the White House.

The Comet, its owner, staff and nearby businesses were caught up in an onslaught of conspiracy theories and fake news during the often contentious presidential campaign and were the victims social media attacks and death threats, the Post said.

"There will be a time and a place to address how and why this happened in greater detail but for now I will simply say we should all condemn the efforts of some people to spread malicious and utterly false accusations about Comet Ping Pong," owner James Alefantis told WUSA-TV.