Washington (AFP) - A senior Democratic congressman sued former president Donald Trump Tuesday, accusing him of violating the 19th century "Ku Klux Klan Act" by supporting the January 6 attack on the US Capitol.

Bennie Thompson accused Trump, his lawyer Rudy Giuliani, and extremist groups the Proud Boys and Oath Keepers of violating the 1871 act by supporting efforts to stop Congress from certifying Joe Biden as the new US president.

Thompson, who is Black and the chairman of the House Homeland Security Committee, cited a law originally created to protect the rights of African Americans after the Civil War and the end of slavery.

The lawsuit was filed in federal court in Washington three days after Trump was acquitted of supporting insurrection in an impeachment trial in the Senate.

While a majority of the Senate, 57 of the 100 members, voted for conviction, it fell short of the two-thirds majority required.

The 1871 law was designed to give the US president powers to oppose violently racist groups like the Ku Klux Klan which sprung up in the wake of the 1861-65 Civil War to oppose equal rights for Black Americans.

One seldom-invoked clause of the act forbids conspiracy to obstruct federal officeholders from performing their jobs.

Thompson alleged that Trump, Giuliani and the two groups conspired "by force, intimidation and threats" to prevent him from discharging his official duty to carry out the certification of Biden's election win.

"The defendants acted in concert to incite and then carry out a riot at the Capitol by promoting an assembly of persons to engage in tumultuous and violent conduct or the threat of it that created grave danger of harm to the Plaintiff and to other Members of Congress," he said.

Attack on Black voter rights

The attack on the Capitol temporarily halted the certification of Biden, left five people dead and scores injured.

In Trump's impeachment trial Democratic prosecutors offered strong evidence to show that the attack was the outcome of two months of deliberate incitement and encouragement by Trump.

It culminated in the January 6 rally outside the White House attended by members of the two groups, in which both Trump and Giuliani directly urged them to march to the Capitol to halt the certification.

Thompson said the Capitol attack arose from a "common plan that the defendants pursued since the election held in November 2020."

The National Association for the Advancement of Colored People (NAACP), a leading civil rights group, is representing Thompson in the suit.

The group said that at least two other members of Congress, both also African-American, would join the action as plaintiffs.

"The events on January 6th were just one more attempt by Donald Trump and his allies to make sure that African-American voters were disenfranchised -- this time, by trying to stop members of Congress from doing their job and certifying the election results," the NAACP said.

Thompson, a native of Mississippi where he grew up under the pervasive threat of Ku Klux Klan violence, is seeking unspecified damages for "emotional distress" and  punitive damages to punish Trump and the other defendants for "unlawful conduct."

Trump spokesman Jason Miller rejected the allegations in the suit.

"President Trump did not plan, produce or organize the Jan. 6 rally... President Trump did not incite or conspire to incite any violence at the Capitol on Jan. 6," Miller told AFP in a statement.

There was no immediate comment from Giuliani.