A man who threatened to set off a bomb on Washington's Capitol Hill surrendered to police Thursday, ending an hours-long standoff that rattled lawmakers still shaken by January's pro-Trump insurrection.

The incident triggered evacuations and massive police response and sent jolts of anxiety through a city that has spent months on alert, as political tensions soared following the deadly US Capitol riot.

The suspect, an apparent right-wing extremist identified by US Capitol Police as Floyd Ray Roseberry, had been broadcasting live on social media from his truck, assailing President Joe Biden and Democrats, warning of a "revolution," and complaining about the US government and its policy in Afghanistan.

He had also claimed that four more sets of explosives were lying in wait in Washington and that they would be detonated, along with his bomb, if police used deadly force against him.

Much of the complex was cordoned off as police and FBI agents negotiated with the driver, who police said had appeared to have a detonator in his hand.

"He got out of the vehicle and surrendered, and the tactical units that were close by took him into custody without incident," US Capitol Police Chief Thomas Manger told reporters.

Law enforcement were still investigating whether the vehicle, which was parked on the sidewalk next to the Library of Congress, contained actual explosives.

Manger said it was unclear what Roseberry's motives were, but in his Facebook Live steam he issued a series of incoherent threats and asked to speak to Biden.

"I'm trying to get Joe Biden on the phone. I'm parked up here on the sidewalk right beside all this pretty stuff," said the bald-headed man with a salt-and-pepper goatee, wearing a white T-shirt.

"I'm not hurting nobody Joe. I'm not pulling the trigger on this thing. I can't," he said. "I'm telling you, them snipers come in, they start shooting this window out, this bomb's going off."

'The revolution'

The live feed showed Roseberry in his vehicle holding a metal can with clay and a box with buttons and wires on the top, but it was unclear if the contraption was a viable explosive device.

While he did not claim a political affiliation, he referred at one point to "the revolution" and said: "I'm looking for all my other patriots to come out and help me."

Later he lashed out at Biden's party, saying: "You all know what you're doing, Democrats? You're killing America."

Facebook's policy communications director Andy Stone said the profile in question had been removed and the incident was being investigated.

While it remained unclear whether the bomb threat was genuine, the Library of Congress's main buildings were evacuated, as was the nearby US Supreme Court and at least one of the three House office buildings.

The Republican National Committee headquarters was also reportedly evacuated and the Washington Metro's Capitol South subway station closed as a precaution.

A White House official said Biden was briefed on the standoff.

'Threat to every community'

Both the Senate and House of Representatives are currently on recess, but some lawmakers have remained in Washington and staffers are working in the complex.

House Democrat Bill Pascrell offered fierce condemnation of the suspect during the standoff, in comments that echoed the trauma of the insurrection which rocked the Capitol in January.

"A right-wing extremist is now threatening to detonate a bomb at the US Capitol," he tweeted. "Right wing domestic terrorism is a threat to every community in the United States."

Tensions remain high more than seven months after the January 6 riot, when supporters of then-president Donald Trump stormed the US Capitol, fought with police and sought to block certification of the presidential election.

In April, a man rammed a car into Capitol barriers, killing one officer before the attacker was shot and killed.

The January uprising prompted authorities to erect tall metal fencing and razor wire around the Capitol complex, which only came down in July.