A group of seven Capitol Police officers filed a lawsuit on Thursday accusing former President Donald J. Trump and nearly 20 members of far-right extremist groups and political organizations of a plot to disrupt the peaceful transition of power during the Capitol riot on Jan. 6.

The suit, which implicated members of the Proud Boys, the Oath Keepers militia and Trump associates like Roger J. Stone Jr., was arguably the most expansive civil effort to date seeking to hold Mr. Trump and his allies legally accountable for the storming of the Capitol.