The ashes of China's late Nobel laureate Liu Xiaobo were scattered in the sea Saturday after a controversial funeral, as his friends worried about the fate of the democracy advocate's widow.

Officials showed a video in which his wife, Liu Xia, and others lowered a white circular urn into the water, two days after the democracy advocate died of liver cancer aged 61 while in custody.

The sea burial deprives family and supporters of a physical place to pay respects to a writer whose calls for political reform angered the Communist regime and led to his arrest in 2008.

His older brother, Liu Xiaoguang, paid tribute to the Communist Party and thanked officials for their "humanistic care" as he spoke at a news conference orchestrated by the authorities in the northeastern city of Shenyang, where Liu Xiaobo died on Thursday.

Authorities have tightly controlled information about Liu Xiaobo's health and life as well as access to his family members.

Liu Xiaoguang said Liu Xia -- who has been under house arrest since 2010 and has yet to appear in public since his death -- was in "weak condition" and experiencing such "great sorrow" and that she may need to be treated in hospital.

Liu Xiaobo's body was cremated "in accordance with the will of his family members and local customs", said Zhang Qingyang, an official from the Shenyang municipal office.

Officials released photos showing Liu Xia with her brother, and two of Liu Xiaobo's brothers in front of the body, which was covered with white petals and surrounded by flowers at a funeral home.

Zhang also said "friends" were at the ceremony.

But Amnesty International's China researcher Patrick Poon told AFP that he did not recognise any of the row of non-family members in the official photo and people close to the Liu couple identified at least one "state security police officer" among them.

Chinese dissident artist critic Ai Weiwei, who lives in Berlin tweeted a photo of the funeral and called the display "disgusting" and a "violation" of the deceased.

China's government faced a global backlash for denying Liu Xiaobo's wish to be treated abroad, and the United States and European Union have called on the government to release Liu Xia and let her leave China.

"As far as I know, Liu Xia is in a free condition," municipal official Zhang said, though friends cast doubt that she was released.

Jared Genser, a US lawyer who represented the Nobel Peace Prize winner, said she has been held "incommunicado" since her husband's death.

"The most preposterous thing is that even during his cremation and funeral he still was not free. And now it's been passed on to his wife, who will continue to lead on that same freedom-less existence," Hu Jia, a Beijing-based activist and family friend, told AFP.

Rescue her 'fast'

Liu was jailed in 2008 after co-writing a petition calling for democratic reforms. The veteran of the 1989 Tiananmen Square pro-democracy protests was sentenced to 11 years in prison for "subversion" a year later.

He died of multiple organ failure at a Shenyang hospital more than a month after he was transferred from prison due to late-stage liver cancer.

At his funeral, Mozart's Requiem was played and Liu Xia "fixed her eyes on him a long time, mumbling to say farewell," Zhang said, adding that she was "in very low spirits".

"She has just lost her husband, so she is currently emotionally grieving," Zhang said.

"It's best for her not to receive too much outside interference during this period after Liu Xiaobo has died," he said.

Genser said Liu Xia has been held for seven years even though she has never been charged with any crime.

"The world needs to mobilise to rescue her - and fast," he said in a statement.

Another family friend and activist, Ye Du, also cast doubt that she was released: "Liu Xia has never been free. There's no doubt about that."

The foreign ministry lashed out at the international criticism on Friday, saying it lodged official protests with the United States, Germany, France and the United Nations human rights office.

The foreign ministry said giving Liu the Nobel Peace Prize was "blasphemy" while state-run media has called him a "convicted criminal".

"His influence has breached the fundamental moral principle of Chinese patriotism and posed a challenge to China's stability and national security," the nationalist Global Times tabloid said in an editorial. "This is why the Chinese society opposes and despises him."