Sara Jane Moore, who fired a handgun at US President Gerald Ford in an attempted assassination outside a San Francisco hotel in 1975, has died, according to US media.

Moore, who was sentenced to life in prison but was freed on parole in her final years, was 95.

Her attack came just days after Ford, a Republican, was targeted in an unrelated murder plot by associates of cult leader Charles Manson.

Ford was not injured in Moore's attack, which she said was intended to spark an American revolution. Her death comes amid renewed attention to presidential security following two attempts on President Donald Trump's life.

On 22 September 1975, Moore fired a .38-caliber revolver she had purchased hours earlier at Ford, but missed.

A former US Marine standing nearby her in the crowd of some 3,000 people then subdued her, forcing her to miss her second shot.

She had been arrested a day earlier after a security official spotted her with a gun at a crowd gathered for Ford. That gun was confiscated, leading her to purchase another gun.

The mother-of-four from California pleaded guilty and was sentenced to life in prison. She served 32 years before she was released on parole.

Her attack on Ford occurred just 17 days after Manson follower Lynette Squeaky Fromme pointed a gun at the then-president but failed to fire a shot before she was apprehended.

Moore's death came just days after the 50th anniversary of her attempt.

In a 2009 interview with NBC News, Moore said she had become radicalized by social upheaval in the 1960s and 1970s.

It was a time that people don't remember. You know we had a war... the Vietnam War, you became, I became immersed in it. We were saying the country needed to change, she said.

The only way it was going to change was a violent revolution. I genuinely thought that [shooting Ford] might trigger that new revolution in this country.

Investigators found no connections with revolutionary groups, and judged her to be legally sane.

On Tuesday, Ryan Routh was found guilty of attempting to assassinate Trump on a Florida golf course in September last year.

He will be sentenced on 18 December and is facing up to life in prison.