Salman Rushdie's agent has said "the news is not good" after the author was stabbed at an event in New York state.
He was attacked on stage, and is now on a ventilator and unable to speak, Andrew Wylie said in a statement, adding that the author will lose one eye.
Mr Rushdie has suffered years of Islamist death threats after writing The Satanic Verses, published in 1988.
Police detained a suspect named as Hadi Matar, 24, from Fairview, New Jersey.
New York State Police said the suspect ran onto the stage and attacked Mr Rushdie and an interviewer at an event at the Chautauqua Institution in western New York state.
"Salman will likely lose one eye; the nerves in his arm were severed; and his liver was stabbed and damaged," his agent, Andrew Wylie said.
No motive or charges have yet been confirmed by police, who are in the process of obtaining search warrants to examine a backpack and electronic devices found at the centre.
Mr Rushdie was stabbed at least once in the neck, and at least once in the abdomen, authorities said. He was taken to a hospital in Erie, Pennsylvania, by helicopter.
The interviewer who was with him on stage, Henry Reese, suffered a minor head injury and was taken to a local hospital. Mr Reese is the co-founder of a non-profit that provides sanctuary to writers exiled under threat of persecution.
Police told a press conference that staff and audience members had rushed the attacker and took him to the ground, and he was then arrested.
Linda Abrams, an onlooker from the city of Buffalo, told The New York Times that the assailant kept trying to attack Mr Rushdie after he was restrained.
"It took like five men to pull him away and he was still stabbing," Ms Abrams said. "He was just furious, furious. Like intensely strong and just fast."
Salman Rushdie: The writer who emerged from hiding
Another onlooker, Rita Landman, told the paper that Mr Rushdie appeared to be alive immediately after the attack.
"People were saying, 'He has a pulse, he has a pulse, he has a pulse'," she said.
A video posted online shows the moment people rushed onto the stage to restrain the attacker and help the injured men. Police said a doctor in the audience gave Mr Rushdie first aid.
Indian-born novelist Mr Rushdie catapulted to fame with Midnight's Children in 1981, which went on to sell over one million copies in the UK alone.
But his fourth book, in 1988 - The Satanic Verses - forced him into hiding for nine years.
The surrealist, post-modern novel sparked outrage among some Muslims, who considered its content to be blasphemous, and was banned in some countries.
A year after the book's release, Iran's Supreme Leader Ayatollah Khomeini called for Mr Rushdie's execution. He offered a $3m (£2.5m) reward in a fatwa - a legal decree issued by an Islamic religious leader.
The bounty over Mr Rushdie's head remains active, and although Iran's government has distanced itself from Khomeini's decree, a quasi-official Iranian religious foundation added a further $500,000 to the reward in 2012.
The British-American citizen - who was born to non-practising Muslims and is an atheist himself - has become a vocal advocate for freedom of expression, defending his work on several occasions.
Salman Rushdie has faced death threats for more than 30 years since the publication of The Satanic Verses. Mr Rushdie said the main thrust of his novel was to examine the immigrant experience, but some Muslims were offended by portrayals of the Prophet Muhammed and the questioning of the nature of the revelation of the Quran as the word of God.
The Satanic Verses was banned first in the author's country of birth, India, and then several other countries before Iran's Ayatollah Khomeini issued his infamous fatwa.
The fatwa called for the killing of anyone involved in the publication of the book and offered rewards to those who took part in the murders. That fatwa has never formally been rescinded.
Surprised by the widespread nature of the protests, Salman Rushdie apologised to Muslims but went into hiding for nearly 10 years.
Though the writer had escaped physical harm until now, others associated with the book had not, with its Norwegian publisher having been shot and seriously wounded in the early 90s, and its Japanese translator stabbed to death.
When Mr Rushdie was knighted in 2007 by the Queen, it sparked protests in Iran and Pakistan, where one cabinet minister said the honour "justifies suicide attacks".
Several literary events attended by Mr Rushdie have been subject to threats and boycotts - but he continues to write. His next novel, Victory City, is due to be published in February 2023.
His appearance at the Chautauqua Institution event, in western New York, was the first in a summertime lecture series hosted by the non-profit organisation.
UK Prime Minister Boris Johnson tweeted: "Appalled that Sir Salman Rushdie has been stabbed while exercising a right we should never cease to defend."
French President Emmanuel Macron wrote that Mr Rushdie "embodied freedom and the fight against obscurantism" and was the victim "of a cowardly attack by the forces of hatred and barbarism".
Writer and graphic novel creator Neil Gaiman said he was "shocked and distressed" by the attack on his friend and fellow writer.
"He's a good man and a brilliant one and I hope he's okay," Gaiman wrote on Twitter.
In a statement, Mr Rushdie's publishers at Penguin Random House said: "We condemn this violent public assault, and our thoughts are with Salman and his family at this distressing time."
And New York Governor Kathy Hochul vowed to "assist however needed in the investigation" into the stabbing.
"Here's an individual who has spent decades speaking truth to power. Someone who's been out there, unafraid, despite the threats that have followed him his entire adult life," she said.