Several hundred demonstrators gathered Thursday in New York's Times Square and chanted slogans including "Hands off Mueller" and "Nobody's above the law" before marching downtown.
In Chicago, Democratic Sen. Dick Durbin joined several hundred protesters at Federal Plaza.
Crowds also turned out at the White House and in Greensboro, North Carolina; Chattanooga, Tennessee; Las Vegas and many other places.
Organizers say the naming of acting Attorney General Matthew Whitaker is a "deliberate attempt to obstruct the special counsel's investigation."
Trump asked for Jeff Sessions' resignation and then replaced him with Whitaker, his chief of staff. Whitaker has criticized Mueller's probe.
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