Well before the concrete blocks and double row of tall fences that ring the US Capitol, National Guard troops with automatic weapons check vehicles.

They are under strict orders to take no chances.

The inauguration of former vice-president Joe Biden as the 46th president of the United States on Wednesday will take place amid the tightest security in living memory.

Referring to the Jan 6 storming of the US Capitol by a mob of President Donald Trump's supporters, Washington Mayor Muriel Bowser told a press conference on Friday: "We saw white extremists storm the Capitol building who were trained and organised. We all have to think about a new posture."

That posture is to turn the heart of the federal government into a fortress, complete with militarised red and green zones - red for maximum restrictions.

"We continue to ask anyone who doesn't absolutely need to be out and about, or in those restricted areas to avoid them," said Ms Bowser.

So far, about 21,000 armed National Guard troops - the number may grow to 25,000 - have been deployed to lock down the capital ahead of the inauguration, which has been drastically reduced in size and scale because of the coronavirus pandemic and now the security concerns.

A large part of the historic National Mall is already closed. Some train metro stations shut down on Friday.

Across the country, too, from Phoenix, Arizona, in the south-west to Lansing, Michigan, in the north-east, tall chain-link fences have been erected around state capitals.

The Federal Bureau of Investigation (FBI) has warned law enforcement agencies of planned rallies at all 50 state capitals, as well as in Washington, DC from today until the inauguration.

Many of these will be armed protests.

Michigan Governor Gretchen Whitmer, herself the target recently of a kidnapping and murder plot by a group of white supremacists, foiled by the FBI, has activated the Michigan National Guard to help with security.

Law enforcement agencies are beefing up airport security and vetting airline passenger lists, as right-wing groups emboldened by the storming of the US Capitol answer calls to travel for protests. Some are planning to turn up armed.

The Transportation Security Administration is increasing the number of federal marshals on flights and explosive-detection dogs at airports, the New York Times said.

And security experts are warning that the violent storming of the Capitol by Trump supporters may be just the beginning of more conflict, and well beyond this week.

"Massive quantities of weapons have been exchanging hands on public forums and in response to a prospective insurrection," security analyst Malcolm Nance told a congressional hearing last Wednesday.

"This is only the beginning," he said, predicting that following the Trump presidency, the conflict will "transition into an active insurgency".

Separately, Mr Ali Soufan, chairman and chief executive officer of the global security consultancy Soufan Group, on Thursday said white supremacists have been "allowed to operate and function with... impunity and the chickens are coming home to roost".

"This is not only about the people who attacked (the Capitol) but also about the network that allowed them to operate so openly... that now they are actually, literally, threatening 50 capitals in the United States," he told a virtual forum. "You have to target the network."

During a press conference last Tuesday, acting US attorney for the District of Columbia Michael Sherwin said prosecutors looking into the Capitol riot are treating the matter as a "significant counter-terrorism or counter-intelligence investigation" involving probes into "money, travel records, disposition, movement, communication records".

One additional factor causing concern is the number of active or former police or military involved in storming the Capitol. Several groups acted with military-like intent and coordination.

The Pentagon's internal watchdog on Thursday said it would investigate whether the US Department of Defence has adequate procedures in place to prevent white supremacists and other extremists from joining and remaining in the military.