WASHINGTON — A group of Democratic operatives were repairing to a luxury golf resort near Miami on Friday to plot an opposition strategy to Donald J. Trump. Scores of others were deserting the nation’s capital for tropical beaches or woodland cabins without internet access, the better to distract themselves from the swearing-in.

But for many of the Democratic activists who remained in Washington, Mr. Trump’s inauguration was a time to ratchet up public protest and dissent against his presidency.

On a day that is traditionally a celebration of national unity that transcends party politics, Democrats are taking battle stations for what they expect to be a yearslong effort to oppose the new commander in chief.

“We want to stand firm in the face of injustice, and we want to let Donald Trump and our entire federal government know, as well as state governments, that we are determined to see things changed,” said Lauren Footman, 25, a community organizer who is staging an afternoon Rally for Humanity at the Martin Luther King Jr. Memorial to resist what she called the “forces of intolerance.”

“This is about saying to the top leadership of this country, ‘We are watching you, we are ready for the next four years and we’re gearing up to take action,’” Ms. Footman said.

On the campus of George Washington University, a few blocks from the White House, a group of activists planned an eight-hour “resistance training workshop” designed to teach liberal millennials techniques to thwart Mr. Trump, by lobbying Congress, making media appearances or running for office.