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 affiliated with a group called Disrupt J20 intermittently blocked multiple security checkpoints leading to the largest public viewing area for the ceremony, and several were led away by police clad in riot helmets and body armor.
Another group of about 150 protesters dressed in black, some of whom wore masks, marched through the city, smashing shop windows and car windows and rolling garbage cans and newspaper vending boxes into the street to form barriers.
One of the signs carried by the group read "Make Racists Afraid Again."
The Disrupt J20 protest organizer Alli McCracken, 28, of Washington, said the group was voicing 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 US imperialism and we feel Trump will continue that legacy," McCracken said on a gray morning with intermittent light rain.
Trump supporters appeared unfazed by the protests.
"We all have First Amendment rights, so I don't really care about the protest as long as nobody touches me," said Bob Burns, 56, of Rosedale, Maryland, referring to the US Constitution's free speech guarantee. He wore military ribbons pinned to his baseball cap.
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 US Navy Memorial, along the parade route.
Trump supporters flooded into the capital, many sporting shirts and hats bearing his "Make America Great Again" campaign slogan.
Carl Beams, 36, from Howell, New Jersey, stood in line with thousands of Trump supporters waiting to enter the National Mall to view the midday (1700 GMT) inauguration.
"This is a great moment in history. I wanted to be able to say I was here firsthand," said Beams, who runs a martial-arts school.
He said he believed that Trump could be a unifying force: "I think he's sending the right message and doing his part to make that happen."
Bikers, potheads
Other groups planning to be on the streets on Friday include Bikers for Trump, which will hold a parade route rally without 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.
Activists in London hung a banner reading "Build bridges not walls" on the city's iconic Tower Bridge on Friday, a reference to Trump's signature campaign promise of building a wall on the US-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 US 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-kilometre) area of downtown Washington
Security officials aimed to keep the Trump supporters and opponents separate, using similar tactics employed during last year's Republican and Democratic political conventions.
"If you come down to the event, you are going to see some people who have views who are different than yours," said the city's interim chief of police, Peter Newsham. "But one thing we can agree on in Washington is that we can agree to disagree."