The body of China's Nobel laureate Liu Xiaobo has been cremated after a private ceremony attended by his wife and friends.

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

Officials released photos showing his wife, Liu Xia, with her brother, Liu Xiaobo's brother and friends in front of the body surrounded by white flowers at a funeral home.

Mozart's Requiem was played at the funeral, Zhang said.

Liu, a prominent dissident since the 1989 Tiananmen Square pro-democracy protests, died from multiple organ failure at a Shenyang hospital on Thursday at age 61, more than a month after he was transferred from prison due to late-stage liver cancer.

China's government has come under international criticism after it ignored pleas to let Liu get treatment abroad as he had wished.

It now faces calls to free Liu's wife from the strict house arrest she has lived under for years even though she has not been convicted of any crime.

"As far as I know, Liu Xia is in a free condition," Zhang said, though it was unclear whether she was released.

"She has just lost her husband, so she is currently emotionally grieving," he said, adding that it was best for "her not to receive too much outside interference during this period".

However, Jared Genser, a US lawyer who represented Liu Xiaobo, said Liu Xia has been held "incommunicado" since his death.

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

OBITUARY: Liu Xiaobo

Tributes have rolled in from around the world to mourn Liu, but there is little mention of him in China's heavily censored state media and social networking platforms.

Liu was jailed in 2008 after co-writing a petition calling for democratic reforms. He was sentenced to 11 years in prison for "subversion" a year later.

He was awarded the Nobel Peace Prize in 2010 and was represented by an empty chair at the ceremony in Oslo.

The leader of the Norwegian Nobel Committee which, to Beijing's ire, awarded Liu the Peace Prize, said the Chinese government bore a heavy responsibility for his death.

The ruling Communist Party considered Liu's writings subversive and imprisoned him four times.