With footage of a pro-Trump mob storming the US Capitol playing out in their chamber, US senators sat transfixed before the shocking images that dominated the start of the former president's impeachment trial.

Despite the sergeant at arm's command that all 100 senators remain silent "on pain of imprisonment," an informal, detached mood descended Tuesday on the Senate's Republican side, where many lawmakers have said they oppose holding the unprecedented second trial of Donald Trump.

Chitter-chatter swirled. Marco Rubio flipped non-committally through a thick book. A sockless Richard Burr, bracing for a marathon session, stuffed his desk with snacks.

Freshman Senator Cynthia Lummis, perhaps not having received the memo about coronavirus precautions, sat maskless for 30 minutes as she pored over documents.

Even as top Senate Democrat Chuck Schumer opened the proceedings by describing the incitement of insurrection allegation against Trump as "the gravest charges ever brought against a president of the United States," many Republicans met the declaration with a collective shrug.

And conservative Senator Tom Cotton made a point of working on a spreadsheet instead of focusing on the debate about the constitutionality of the trial.

But when Democratic impeachment manager Jamie Raskin played a 13-minute video montage depicting some of the most violent clashes of the deadly January 6 siege, the chamber fell under a hush, with senators riveted by the graphic images and chaotic audio.

Republicans and Democrats alike sat rigid as some of the coarsest epithets ever to color a Senate proceeding rang out in the chamber.

Then, they watched the chilling images of a self-described QAnon shaman and other invaders taking over the Senate itself after many of the lawmakers -- the same ones sitting as jurors Tuesday -- fled for their lives.

"It's not very often you have a trial at the scene of the crime," Democratic Senator Chris Murphy told AFP shortly before the proceedings began, speaking of the emotion that could weigh on senators as they sit in judgment of Trump.

"It was our chamber that was violated," he added.

"We were all party to the insurrection, we experienced it first-hand," said another Democrat, Senator Ben Cardin. "It's something that's personal to those of us who were in harm's way."

Not 'our future'

But Republican Senator Bill Cassidy said he would not be caught up in such passions.

"The settings in which we hold the trial do not make (the events of January 6) more vivid for me," Cassidy said. "They're vivid enough in and of themselves."

They were for Raskin, too. The congressman from Maryland, managing the arguments against Trump just six weeks after his son died by suicide, choked up as he recalled how his daughter and his son-in-law, visiting the Capitol that day, "thought they were going to die."

Raskin pointed to the multiple deaths and gruesome injuries suffered during the insurrection.

"People's eyes were gouged, an officer had a heart attack, an officer lost three fingers that day," he said, before adding that two officers have since taken their own lives.

"Senators, this cannot be our future," he said, imploring the chamber not to ordain a "January exception" that would allow a US president to avoid punishment in the final weeks of his terms.

"This cannot be the future of America."

Meanwhile the security repercussions of January 6 remained. The US Capitol remained on effective lockdown, with metal fencing and heavily armed National Guard members surrounding the building.

Inside the chamber, the public galleries -- where a man carrying zip-ties that can be used to detain or kidnap people was photographed in a chilling January 6 image -- were all but empty, as everyday citizens were barred from attending.