Troops in riot gear lined the streets of Washington DC and armoured vehicles and concrete barriers blocked the empty streets around the US Capitol on Wednesday.

The city was on edge, two weeks after pro-Trump rioters stormed the US Capitol in a deadly riot.

But there were no crowds, and President Biden and Vice President Kamala Harris were safely sworn in during the ceremony.

"Just days after a riotous mob thought they could use violence to silence the will of the people, to stop the work of our democracy, to drive us from this sacred ground, it did not happen," Biden said.

"It will never happen. Not today. Not tomorrow, not ever, not ever."

There were a few scattered arrests, but no serious disruptions in the city during the ceremony.

More than 25,000 troops and police were called in for duty ahead of the ceremony, while checkpoints were set up at intersections.

In their preparations, law enforcement officials not only contended with the potential for outside threats but also with rising concerns about an insider attack.

In the hours before the event, federal agents were monitoring "concerning online chatter," which included an array of threats against elected officials and discussions about ways to infiltrate the inauguration, an official said.

The FBI also warned law enforcement officials about the possibility that members of right-wing fringe groups could pose as National Guard troops, according to two law enforcement officials familiar with the matter.

The increased security is likely to remain in the nation's capital for at least a few more days.

Australian Associated Press