With his hand on the Bible, Donald J. Trump has pledged to "preserve, protect and defend the Constitution of the United States," taking the formal oath of office and completing an unlikely trajectory from real-estate mogul to the nation's 45th president.

A crowd that stretched along the National Mall assembled under gray, Washington skies to witness the ceremony. So did nearly every living former president, from Jimmy Carter and George W. Bush to Bill Clinton and his wife, Hillary, who was defeated by Trump last November after a costly and divisive campaign.

"We, the citizens of America, are now joined in a great national effort," Trump said. "We are transferring power from Washington D.C., and giving it back to you, the people."

Trump used his 16-minute remarks to promise an era of populist change, even as he was surrounded by members of Congress and all eight sitting members of the Supreme Court.

"For too long, a small group in our nation's capital has reaped the rewards of government while the people have borne the cost," Trump said. "Washington flourished but the people did not share in its wealth. The establishment protected itself."

He criticized an educational system "flush with cash" that fails to fulfill its mission to students. He lamented factories left to rust while jobs flowed overseas. And he decried crime and gang violence, despite reports from the FBI and criminologists that the U.S. crime rate remains near historic lows.

"This American carnage stops right here and stops right now," Trump said, amid cheers from the crowd.

Soon, Trump and his vice president, former Indiana Gov. Mike Pence, will travel a little under two miles from the U.S. Capitol to the White House, part of a parade that includes 8,000 members of the military, school marching bands, the Boy Scouts and veterans groups.

Also lining their route: Protesters representing 99 different groups. In sporadic exchanges at security check points across downtown Washington, protesters shouted: "No Trump! No KKK! No fascist USA!" Others carried signs with such slogans as "Fake 45."

But, so far, skirmishes appeared to be mostly free from violence. Not far from the White House, several hundred protesters clad in black ran through the streets, breaking windows and knocking over trash cans. Police fired pepper spray to subdue demonstrators.

Preparations for the event have been under way for months. Around 28,000 officers are on hand to provide security for the event, including Secret Service, FBI agents, the Coast Guard, police departments, and nearly 8,000 National Guard troops. Homeland Security Secretary Jeh Johnson said authorities had "no specific, credible threat" directed at the proceedings and authorities reported few, if any, arrests.

Meanwhile, at the White House, household crews are furiously preparing the mansion for its new occupants. Earlier in the day, camera crews spotted President Obama making one last visit to the Oval Office, slipping a letter in the desk drawer for his successor.

Staff members at the residence presented the Obamas with two flags: one that welcomed them to the White House eight years ago; another that flew over the building Friday morning, on their last day there. The Obama family heads to Palm Springs, California, Friday for a vacation.

The formal hand-off of power is now complete, as the Trump administration only begins to come into focus. Trump, who has had no experience in elective office, now controls a vast federal bureaucracy, including national security, international diplomacy, the economy, the environment and the justice system. He's painted his governing agenda in broad strokes, promising to "Make America Great Again" by bringing jobs back to the U.S. and deliver a tough message on the international stage.

His formal remarks from the inaugural platform hearkened back to his campaign speeches: "We will bring back our jobs. We will bring back our borders. We will bring back our wealth and we will bring back our dreams."

 Agencies