About 900,000 people were expected to pack the grassy National Mall facing the Capitol, where Trump will be sworn in, as well as the parade route along Pennsylvania Avenue to the White House and other parts of central Washington.
Protesters organized by a group called Disrupt J20 linked arms at one of the dozen security checkpoints leading to the largest public viewing area for the ceremony, and several were led away by police in riot helmets and body armor.
Organizer Alli McCracken, 28, of Washington, said Disrupt J20 hoped to shut down the checkpoint as a sign of its displeasure over Trump's controversial comments about women, illegal immigrants and Muslims.
"We have a lot of people of diverse backgrounds who are against U.S. imperialism and we feel Trump will continue that legacy," McCracken said on a gray morning when rain was forecast.
One of the largest anti-Trump protests expected on Friday will be organized by the ANSWER Coalition, a broad-based liberal group, which expects to have thousands at the U.S. Navy Memorial, along the parade route.
"It's Day One, we're saying, of a larger era of resistance, and we believe we're going to send a very powerful message to Trump and the government," said Ben Becker, 33, an organizer with the group.
Trump supporters also flooded into the capital, many sporting baseball caps bearing his "Make America Great Again" campaign slogan.
One of them, Kathy Williams, 54, a Veterans Administration hospice nurse from Cleveland, said she woke up around 4 a.m. to be at the event. Trump, a real estate developer and former reality TV star, is expected to be sworn in at midday (1700 GMT).
"God is taking our country back because Donald Trump is going to put God back in our country and let Christians be equal with everybody again. I support our military, that's why I voted for Trump," said Williams, who called herself a "Gun- and bible-carrying God-fearing Republican."
BIKERS, POTHEADS
Other groups planning to be on the streets on Friday include Bikers for Trump, who will hold a parade route rally without their motorcycles, and pro-marijuana activists who plan to hand out 4,200 joints to be lit up in violation of federal and local laws.
While Washington will be the focal point of the protests, anti-Trump activists have planned sympathy rallies around the nation and the world.
Thousands of people in New York marched to Trump Tower, the businessman's home, on Thursday night to protest his victory in the Nov. 8 election. Activists in London hung a banner reading "Build bridges not walls" on the city's iconic Tower Bridge on Friday, in a reference to Trump's signature campaign promise of building a wall on the U.S.-Mexican border.
About 30 groups have obtained permits for protests they estimate will attract about 270,000 people on Friday and Saturday, far more than have been seen in other recent presidential inaugurations.
By far the largest protest is expected to be Saturday's Women's March on Washington, which some 200,000 people from around the country are expected to attend.
The U.S. Secret Service, Washington police and other law enforcement agencies planned to have some 28,000 officers in place to secure a roughly three-square-mile (almost eight-square-kilometer) area of downtown Washington
U.S. Department of Homeland Security Secretary Jeh Johnson said police aimed to keep groups separate, using similar tactics employed during last year's Republican and Democratic political conventions.
(Additional reporting by Joanathan Landay; Editing by Jonathan Oatis and Paul Simao)
