Metropolitan Police Department Chief Robert J. Contee said 47 of the 52 arrests were related to violations of Mayor Muriel Bowser's curfew, with 26 of those involving people arrested on US Capitol grounds.

Several others were arrested on charges related to carrying unlicensed or prohibited firearms.

In addition, Contee said, two pipe bombs were recovered from the headquarters of the Republican and Democratic national committees, as well as a cooler from a vehicle on US Capitol grounds that contained Molotov cocktails.

Contee declined to identify the woman, who was shot and killed by a Capitol Police officer, saying next of kin notification was still pending.

Three other people also died on Wednesday because of medical emergencies, he said, and 14 police officers were injured - two of whom remain hospitalised.

It was not clear if other federal or local police agencies, including the Capitol Police, had made additional arrests.

While the number of people arrested is expected to grow, the initial number pales in comparison to the more than 300 people who were arrested by police following the June 1 protests in the district related to the police killing of George Floyd.

In that incident, baton-swinging police and federal agents fired smoke canisters, flashbang grenades and rubber bullets to drive protesters farther from the White House, enabling President Donald Trump to walk across Lafayette Park and hold up a Bible in front of St. John's Church.

While there was staunch criticism for being too aggressive at Lafayette Square, police are now facing questions about why they did not do more to secure the Capitol and let many of the rioters later exit the building without making arrests.

Late on Wednesday, the FBI asked the public to help agents identify people were "actively instigating violence".

Mayor Bowser extended a public emergency declaration for 15 days, an action she said will allow her to restrict peoples' movements around the city if necessary.

Australian Associated Press