President-elect Donald J. Trump delivered a boastful, campaign-style rally at a downtown Washington arena on the eve of his second inaugural, celebrating his election victory and vowing to advance his agenda in spite of what he called a “failed and corrupt political establishment” in the nation’s capital.

The speech at Capital One Arena down the street from the White House was classic Trump. In remarks laced with exaggerations and outright falsehoods, the president-elect railed about illegal immigration, bragged about the swing states he won last November, and denigrated President Biden.

“Tomorrow at noon, the curtain closes on four long years of American decline, and we begin a brand-new day of American strength and prosperity, dignity and pride,” the president-elect said, adding that Mr. Biden presided over “a failed administration. We’re not going to take it anymore.”

The rally was a break with the tradition for presidents, who have sought to reserve their comments for the official inauguration speech, to be delivered moments after taking the oath of office with the world watching.

In one of his most hotly anticipated actions, Mr. Trump hinted strongly that he would pardon many of the people convicted of attacking the Capitol on Jan. 6 four years ago.

“Tomorrow, everybody in this very large arena will be very happy with my decision on the J6 hostages,” he said, using the term that he prefers for the people who have been convicted in the attack. “You’ll be very, very happy. I would say about 99.9 percent in this beautiful arena.”

He was joined on the stage by Village People for a live rendition of their song “Y.M.C.A.,” which has become part of the unofficial soundtrack of Mr. Trump’s campaign because of his repeated use of it at his rallies.

Mr. Trump will not take office until Monday at noon. But his return to Washington began in earnest on Sunday, with a full day of ceremonial and political events intended to underscore his remarkable ascendance — again — to the seat of American power.

For Mr. Trump, the rally was an opportunity to speak to his supporters in the language they have grown to love: a casual and sometimes rambling review of his own accomplishments, filled with attacks on his adversaries — journalists, Democrats, immigrants, moderate Republicans and foreign leaders.

Mr. Trump called Elon Musk, the billionaire chief executive of Tesla, to talk about the coming effort to cut government spending and regulations. He vowed to end diversity efforts around the country. And he showed a video recounting deadly attacks on Americans by undocumented immigrants.

“The border security measures I will outline in my inaugural address tomorrow will be the most aggressive, sweeping effort to restore our borders the world has ever seen,” Mr. Trump vowed in front of the boisterous crowd.

“We will stop illegal immigration once and for all,” he added. “We will not be invaded. we will not be occupied, we will not be overrun, we will not be conquered. We will be a free and proud nation once again and that will take place tomorrow at 12 o’clock.”

He also said he would release “in the coming days” the classified records relating to the assassinations of John F. Kennedy, Robert F. Kennedy and Martin Luther King Jr., “and other topics of great public interest. It’s all going to be released.”

It was unclear what, exactly, Mr. Trump was talking about. It was a repeat of a promise he made eight years ago and did not fully make good on.

His speech was filled with the kinds of misleading or exaggerated claims he often made on the campaign trail. At one point on Sunday, the president-elect claimed that he had won the youth vote by 36 points. In fact, exit polls showed that while young voters did shift toward Mr. Trump, he lost most categories of younger voters to Ms. Harris.

A day before he takes the oath of office for the second time, Mr. Trump hosted a private breakfast with Republican senators at Blair House, the 19th-century mansion across the street from the White House that serves as the president’s exclusive guesthouse, often used by world leaders.

He then traveled to Arlington National Cemetery to pay tribute to fallen U.S. service members. At the cemetery, the president-elect placed a wreath at the Tomb of the Unknowns in a somber ceremony that has become a tradition for presidents just before their inaugurals.

Wearing a long overcoat and gloves in the rainy, cold weather, Mr. Trump lifted the large wreath, assisted by a service member at the cemetery, and placed it on an easel at the tomb. He stood silently for a moment before Vice President-elect JD Vance did the same.

After the wreaths were placed, a soldier played “Taps,” the sound echoing through the hallowed burial grounds.

For more than a half-hour, Mr. Trump, Mr. Vance and their wives talked with the families of service members who had been killed at the Abbey Gate in Kabul during the evacuation of American troops from Afghanistan.

An icy rain pelted down as they walked from one gravestone to another in Section 60, where many deceased U.S. veterans of the wars in Iraq and Afghanistan are buried. Mr. Trump placed what appeared to be a gold-tinged challenge coin on each of the headstones. Mrs. Trump and Mrs. Vance carried lilies. Mrs. Trump placed them at each gravesite.

The rally took place blocks from the National Mall, his first such address in Washington since the election in November. The president-elect ended his day by attending a candlelight dinner with supporters.

The busy schedule was a prelude to what Mr. Trump’s aides promise will be an intense day of activity on Monday, including his inaugural address and a flurry of executive orders and presidential actions on immigration and other areas.

Forecasts of frigid weather have scrambled the choreography and cadence of Monday’s inaugural events. Mr. Trump’s inaugural address was moved indoors, to the Capitol, and the traditional parade was canceled, meaning the viewing stand for dignitaries across from the White House on Pennsylvania Avenue, which took months to build, will sit empty.

But Mr. Trump’s aides are racing to rewrite the script. Mr. Trump is planning to return to Capital One Arena on Monday, after he becomes president, and his aides are considering whether to have him sign some of the executive orders from a desk placed onstage.

Even before his Sunday events, Mr. Trump began his day the way he often did while serving as the 45th president: with a post on social media.

“Hostages starting to come out today!” he wrote on his Truth Social site, referring to the implementation of a cease-fire agreement between Israel and Hamas in Gaza. The deal called for the release of hostages, which began on Sunday. He hailed the release of “three wonderful young women.”

Also on Mr. Trump’s mind was the fate of TikTok. He said in a Truth Social post on Sunday morning that he would sign an executive order on Monday to give the Chinese-owned app, which stopped operating in the United States because of a ban that went into effect on Sunday, an extension to keep functioning.

Mr. Trump, who has made clear he wants the app up and running during his inauguration and related events, wrote that there would not be liability for tech companies that keep the app active until he takes office. Soon after, the app began flickering back to life.