Close friends lost touch with the poet Liu Xia after her husband died on Thursday of liver cancer aged 61 while he was in police custody at a hospital in northeast China.
The United States and European Union have called on President Xi Jinping's government to free Liu Xia, who has been under house arrest since 2010, and let her go abroad.
The authorities released images of the grieving wife at a private funeral on Saturday, and later on a boat with relatives as they lowered an urn containing her husband's ashes into the sea.
"We are very worried. We saw from authorities' photos of the funeral that she is weak and pained. She looked like the world's saddest person," said Hu Jia, a Beijing-based activist and close friend.
"If I could see her, I would comfort her and offer her a shoulder to cry on," Hu told AFP.
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'Baseless' house arrest
Following his terminal cancer diagnosis, the democracy advocate requested to receive treatment abroad - a wish that friends believe was in reality for his wife's sake. But the authorities refused to let him go.
Although Liu Xia stayed out of politics, she has been under police watch without charges since shortly after her husband was awarded the Nobel Peace Prize in 2010.
Liu Xiaobo was a veteran of the 1989 Tiananmen Square protests and was detained in 2008 after co-writing Charter 08, a petition calling for democratic reforms.
During the past seven years, Liu Xia was only allowed to leave her Beijing apartment to visit her parents or her husband at his prison in the northeastern province of Liaoning, where he was serving an 11-year sentence until he was admitted at a hospital in early June.
Her father died last year, and her mother died earlier this year.
"There is no higher priority than getting Liu Xia out. That authorities have broadcast their ongoing torture of her heightens the urgency," Human Rights Watch's China director Sophie Richardson said.
"They are broadcasting on multiple mediums and multiple languages images and video of her doing what authorities say she wants to be doing - all the while not letting her speak freely for herself and having held her baselessly under house arrest for years," she told AFP.
"It is an environment shot through with coercion."
Liu Xiaoguang, Liu Xiaobos eldest brother, speaks at a news conference in Shenyang, China. Source: AAP
'Monitored and controlled'
On Saturday, Zhang Qingyang, an official from the Shenyang city municipal office, told reporters Liu Xia was "free", but he did not disclose her whereabouts.
On Friday, Chinese foreign ministry spokesman Geng Shuang had said he had no information about the widow and "will not make prejudgements" about whether she could go overseas.
Another close friend of the Lius, Ye Du, said the last time he reached the family was early Friday, but they sounded nervous and refused to talk about funeral arrangements.
"Liu Xia is definitely monitored and controlled," Ye told AFP, adding that "mourning activities" were also "severely controlled".
Liu Xiaobo's older brother, Liu Xiaoguang, said at a news conference organised by the authorities that the government had followed the family's wishes for the funeral. He also said Liu Xia was so heartbroken that she may need hospital treatment.
But supporters said it was impossible to verify if the family had really wanted a sea burial and noted that the brothers were politically at odds.
Amnesty International China researcher Patrick Poon said Liu Xia, 56, suffers from depression and heart disease.
He said the loss of contact was "strange".
"Why would she suddenly refuse to communicate with her friends when it's a moment she needs others to comfort her great sorrow?"
