WASHINGTON - US Democrats on Saturday were readying for an unprecedented second impeachment of Donald Trump as the defiant president showed no signs of stepping down after the deadly violence at the Capitol.

Democrats said impeachment proceedings could begin as early as Monday -- an extraordinary acceleration of a process that historically has taken weeks, but one that might not be completed before President-elect Joe Biden is sworn into office on January 20.

House Speaker Nancy Pelosi warned that Democrats will launch the process unless Trump resigns or Vice President Mike Pence invokes the 25th Amendment, where the cabinet removes the president.

"He's deranged, unhinged and dangerous. He must go," Pelosi, referring to Trump, tweeted on Friday.

The move to impeach came amid continued fury over the storming of the Capitol on Wednesday by angry Trump supporters, which left five people dead, including a Capitol policeman.

Authorities announced new arrests and charges Saturday over the incursion, including a tattoo-chested man in a horned headdress whose image was beamed around the world.