(Reuters) - Seventeen law firms have signed on with Crowell & Moring to a letter urging U.S. Vice President Mike Pence and the Cabinet to remove President Donald Trump from office by invoking the 25th Amendment, the Washington, D.C.-based firm said Friday.

The signatories include Frank Ryan, the chair of DLA Piper, the third highest-grossing U.S. law firm and one of the largest law firms in the world.

Crowell's management first issued the letter on Thursday, calling on other law firms to join.

"The president has proven himself unfit for office, and a reckless and wanton threat to the Constitution that he pledged to preserve, protect, and defend," the joint letter reads.

Crowell's letter comes two days after a mob of the president's supporters stormed the U.S. Capitol as Congress was meeting to certifying President-elect Joe Biden's victory, forcing lawmakers to flee and leading to at least five deaths in scenes that shocked the world.

Law firm leaders from Amin Talati Wasserman; Baker Thomas Oakley Green; Coburn & Greenbaum; Farella Braun + Martel; Foley Hoag; Hanson Bridgett; Kendall Brill & Kelly; Lewis Baach Kaugmann Middlemiss; Kenneth Reich Law; McNerney & McAullife; Phillips Nizer; Shaheen & Gordon; Sullivan & Worcester; Vandevert Trade Law; Wilson Williams and WTAII PLLC also signed the letter.

At least two other attorney-led groups, Lawyers Defending American Democracy and Lawyers for Good Government, also called for Trump's removal through impeachment or the 25th Amendment. Letters from the groups had attracted more than 15,000 signatures by late Friday afternoon.

Lawyers Defending American Democracy's letter has more than 1,300 signatories, while Lawyers for Good Government collected more than 14,000 signatures from lawyers, other legal professionals and other supporters.

Wednesday's assault on the Capitol quickly led to statements of condemnation from several prominent U.S. lawyers and law firm chieftains, including the leaders of Perkins Coie and Paul, Weiss, Rifkind, Wharton & Garrison and the American Bar Association, though their comments stopped short of demanding the president's immediate ouster.

For the 25th Amendment to be invoked, Pence and the majority of Trump's Cabinet would need to declare that Trump is unable to perform the duties of the presidency. Pence would take over, in that scenario, until Biden is sworn in on Jan. 20.