Trump impeachment trial begins as Chief Justice Roberts, senators sworn in The Senate trial got underway with solemn formality.

The Senate formally accepted the articles of impeachment against President Donald Trump shortly after noon on Thursday, officially triggering only the third trial of a president in U.S. history.

House managers, who will act as prosecutors, arrived on the Senate floor after walking silently across the Capitol to present the articles, with House Intelligence Committee Chairman Adam Schiff taking the lead role in the solemn ritual.

The Senate Sergeant at Arms, following centuries-old tradition, called out: “Hear ye! Hear ye! Hear ye! All persons are commanded to keep silent, on pain of imprisonment, while the House of Representatives is exhibiting to the Senate of the United States articles of impeachment against Donald John Trump, President of the United States.”

Schiff then read the articles accusing the president of abuse of power and obstruction of Congress as all 100 senators sat listening at their desks, some taking notes.

Quoting the articles, he called Trump "a threat to national security" and said his conduct "warrants removal from office."

"Resolved that Donald J Trump is impeached for high crimes and misdemeanors,” Schiff, D-Calif., declared, adding, “Donald J. Trump has abused the presidency.”

Schiff, citing the articles, said President Trump acted with “corrupt motives” by conditioning $391 million in military aid to Ukraine “for the purpose of providing vital military aid” for “corrupt personal benefit.”

“President Trump has abused the power of the presidency in a manner subversive of the Constitution,” Schiff read from the articles, adding that the president “interposed the power of the Presidency” by stonewalling the House impeachment inquiry.

Schiff made note of the officials – by name - who defied House subpoenas for their testimony, including some of the witnesses Senate Democrats now want to testify at the trial, including acting White House Chief of Staff Mick Mulvaney.

Once Schiff finished, the House managers were dismissed and left the Senate chamber.

With the articles "exhibited" -- or read into the record -- Senate Majority Leader Mitch McConnell announced that Chief Justice John Roberts had come from the Supreme Court to be sworn in to preside over trial.

Roberts was sworn in by Republican Sen. Chuck Grassley, the president pro tempore – the highest ranking senator who presides over the Senate in the absence of the vice president.

Once Roberts was sworn in, he administered the same oath he took to all senators -- to do "impartial justice."

“I solemnly swear that in all things appertaining to the trial of the impeachment of President Donald Trump, now pending, I will do impartial justice according to the Constitution and laws: So help me God,” Roberts said, reading from notes.

Each senator then. according to Senate rules and ritual, came forward to sign an "oath book."

ABC's Trish Turner reported the Senate chamber was largely silent as senators signed their names and, unlike the Clinton trial, senators were not taking a ceremonial pen after they wrote out their names.

Most senators were following the rules or decorum, not many had material on their desks, most weren't chatting. GOP. Sen Mike Lee took a Bible from his desk and was reading it.

The only senator absent was Republican Sen. James Inhofe, who was said to be out because of a family medical issue.

After McConnell announced a summons to be issued to the president, and deadlines for his lawyers to file legal briefs responding to the charges, Chief Justice Robert adjourned the Senate, now sitting as a jury in the case, at 2:34 p.m., to reconvene next Tuesday, with Thursday's ceremony lasting about half an hour.

McConnell said earlier in the week that he expects the Senate impeachment trial to kick off in earnest with opening arguments on Tuesday, Jan. 21.

All senators are expected to be in attendance for the duration of the trial, which is expected to last at least two to three weeks, according to several senators.

According to the trial decorum guidelines, when Roberts arrives every single day, senators will have to "silently rise” at their desks and remain standing until Roberts takes his seat.

Senators are expected to do the same when Roberts leaves the chamber every day, as well.

Senators will have to refrain from speaking to their neighboring senators in the chamber and will have to remain seated throughout the duration of the trial every day.

Cellphones and other electronic devices will not be allowed in the chamber -- senators and any staff allowed inside will be asked to keep their devices in a separate room outside the chamber.

Senators will have to refer to Roberts as "Mr. Chief Justice."

Just hours before the Senate proceedings, the Government Accountability Office said it has concluded “that the Office of Management and Budget violated the law when it withheld approximately $214 million appropriated to DOD for security assistance to Ukraine.”

Trump directed that the aid be withheld -- what the GAO called illegal and which is covered in the impeachment articles accusing the president of wrongdoing.

“The law does not permit OMB to withhold funds for policy reasons,” the statement fro the the GAO, an independent, nonpartisan watchdog that investigates requests from members of Congress, read.

The House voted Wednesday afternoon to formally send the impeachment charges against Trump.

The House resolution officially appointed the seven managers, named by House Speaker Nancy Pelosi Wednesday morning.

"This is a very important day for us," Pelosi said Wednesday.

"Time has been our friend in all this," she added, noting what she called the new "incriminating" evidence that has surfaced in the month since the House impeachment vote on Dec. 18, including new documents and other evidence from Lev Parnas, an associate of Rudy Giuliani, the president's personal lawyer.

Schiff said the new evidence, revealed by the House just Tuesday night, must be considered by the Senate.

It’s undecided at this point if the Senate will hear from witnesses or consider new evidence during the trial.

ABC News' Katherine Faulders contributed to this report.