WASHINGTON — Hunter Biden was a habitual user of crack cocaine when he drove his father's Cadillac and illegally bought a gun, his criminal trial has heard as US First Lady Jill Biden looked on in court.

The US president's son is accused of knowingly lying on a form to buy a revolver and ammunition from a Wilmington, Delaware, gun shop on 12 October 2018.

The defense said Biden, 54, was in recovery at the time, so he was truthful when he wrote on the application that he was not a user of illicit drugs.

It is the first time a sitting US president's son has gone on trial.

Biden could face up to 25 years in prison if found guilty of all three federal counts in the case.

He is charged with lying to a federally licensed gun dealer, making a false claim on the application by saying he was not a drug user, and illegally having the gun for 11 days.

On Tuesday, the court heard from the government's first witness, FBI special agent Erika Jensen, who will return to the stand Wednesday. She testified about bank records and texts from Biden's devices that reference drugs.

Through the trial, the jury will hear how Hallie Biden — the widow of the president's late son, Beau Biden — also became addicted to crack cocaine during her brief relationship with Hunter Biden.

She took the gun from him and threw it in a grocery store bin 11 days after he bought it, concerned about what he might do with the weapon.

"No-one is above the law," said prosecutor Derek Hines in Tuesday's opening statement to jurors. "It doesn't matter who you are or what your name is."

Hines said Biden had “crossed the line” when he chose to buy the gun and allegedly lied about a federal background check.

The prosecutor also told jurors that Biden was not accused of any violence, but the jury would hear how guns were "pointed in his face" during drug deals, and he had "learned how to defend himself".

But the defense said this claim had nothing to do with the period when Biden bought the gun in Delaware.

In his opening statement, defense lawyer Abbe Lowell said the Cobra Colt .38 revolver was an impulse buy, under pressure from a pushy gun store owner.

He said his client had driven his father’s Cadillac to a mobile phone store on the day in question before crossing the street to the firearms shop.

At the core of the defense's case is the argument that the federal application form for a gun purchase only asks if the buyer is a drug user — not whether they "have ever been".

Lowell said Biden was in recovery at the time after attending a rehab clinic in California, and did not view himself as an addict.

"Hunter was not using drugs when he bought that gun," he said.

Hunter Biden sat with his defense team, pen in hand, and put his glasses on repeatedly to read documents.

The Yale-educated lawyer often looked at the jury, and appeared to follow proceedings closely.

The US first lady sat directly behind him. President Joe Biden, who has long been highly protective of his son, was at the White House in Washington DC.

The first lady and Hunter Biden's half-sister, Ashley Biden, appeared emotional while listening to recordings aired in court of Hunter describing his descent into addiction. The audio was from his narration of his 2021 memoir Beautiful Things.

Ashley Biden leaned her head against the first lady's as she seemed to wipe her eyes.

At one point, Jill Biden put an arm around the shoulder of Ashley Biden, who later left the Wilmington court and did not return.

The defense said Biden did not deny previous drug abuse, arguing this was a result of repeated tragedy in his life.

This included the death of his mother and baby sister in a 1972 car accident when he was aged three, and the death of his older brother, Beau, from brain cancer in 2015.

Also in court was Biden's wife, Melissa Cohen Biden, who angrily confronted a former Trump aide.

According to a reporter from the BBC's partner CBS, Mrs Cohen Biden launched into a profanity-laced tirade at Garrett Ziegler in the hallway outside the courtroom.

According to other US media, she called Ziegler – who has circulated embarrassing messages and images from a laptop that once belonged to the president's son — "a Nazi".

Biden is currently suing Ziegler for disseminating data from the laptop.

Hunter Biden also faces a trial in California in September on charges of failing to pay $1.4m in taxes. — BBC