WASHINGTON — “Hear ye, hear ye, hear ye. All persons are commanded to keep silent upon pain of imprisonment.”
Those words, an echo of another time, came from Michael C. Stenger, the Senate sergeant-at-arms, and fell upon a hushed Senate chamber just after noon on Thursday. The impeachment trial of Donald John Trump, the 45th president of the United States, on charges of high crimes and misdemeanors was about to begin.
Two hours later, Chief Justice John G. Roberts Jr., in flowing black robes, escorted by four senators, strode into the room to open the third presidential impeachment trial in American history.
“When the chief justice walked in, you could feel the weight of the moment,” Senator Chuck Schumer, Democrat of New York and the minority leader, said later. “I saw members on both sides of the aisle visibly gulp.”
Chief Justice Roberts, who will preside over the trial, was sworn in by Senator Charles E. Grassley, Republican of Iowa and the Senate pro tempore — a post he holds because he is the longest-serving member of the majority.
Mr. Grassley administered the oath, which dates to the 1700s: “I solemnly swear that in all things appertaining to the trial of the impeachment of Donald John Trump, president of the United States, now pending, I will do impartial justice according to the Constitution and laws: So help me God.”
Chief Justice Roberts then administered the same oath to the senators, who rose in unison, raised their right hands and swore to administer “impartial justice.”
One by one, in alphabetical order, the senators then proceeded to the well of the chamber to sign an “oath book” — an affirmation in writing of the vow they had just taken.
In a proceeding laden with history and tradition, the oath book is a relatively recent convention; it dates to 1986, when the Senate updated its impeachment rules for the television era.
Earlier in the day, the seven Democrats appointed by Speaker Nancy Pelosi as “impeachment managers” to prosecute the House’s case walked two-by-two across the Capitol to the Senate chamber. They had already delivered two articles of impeachment, one on abuse of power and the other on obstruction of Congress, to the Senate on Wednesday evening. Now they were invited to “exhibit” — or read them aloud — in the chamber at noon on Thursday.
Representative Adam B. Schiff, Democrat of California and the lead impeachment manager, delivered the formal accusation from the House: President Trump had committed “high crimes and misdemeanors” by withholding military aid from Ukraine to help himself win the 2020 election, and then concealed his actions from Congress.
“President Trump,” Mr. Schiff said, “warrants impeachment and trial, removal from office and disqualification to hold and enjoy any office of honor, trust or profit under the United States.”
There were signs all over the Capitol that this was a different kind of day.
The Capitol Police officers, who have the option to wear turtlenecks, wore ties with their dress blue uniforms. Areas ordinarily open to the news media and visitors were cordoned off with golden stanchions and crimson velvet ropes. Reporters and staff members were issued special color-coded badges — maroon for the news media, brown for Senate staff — to gain entry to the Senate side of the Capitol.
The highly orchestrated rituals of the impeachment trial began on Wednesday, when the House voted, largely along party lines, to transmit the articles to the Senate and appoint the impeachment managers.
Ms. Pelosi needed to sign them, and she did so during a formal “engrossment ceremony,” using more than a dozen black and gold pens that were laid out on silver trays. (Later she gave the pens as mementos to committee leaders and other Democrats who played a role in the House impeachment inquiry.)
Mr. Schiff and his team then walked slowly in a procession through the Capitol, through Statuary Hall and the Rotunda, to deliver the articles to the Senate.
Mr. Trump’s trial will reconvene at 1 p.m. Tuesday. On his way out of the Capitol, Senator John Cornyn, Republican of Texas, reflected on what was going through his mind as the chief justice administered the oath.
“The gravity of what we are undertaking, I think, is sinking in for all of us,” he said.