The United States and the European Union urged President Xi Jinping's government to let Liu's widow, the poet Liu Xia, who has been under house arrest since 2010, leave the country.
Chinese doctors said she was by her husband's side when he lost his battle with liver cancer on Thursday at age 61, more than a month after he was transferred from prison to a hospital in the northeastern city of Shenyang.
Liu's main doctor said he was able to say goodbye to his wife and in his final moments told her to "live well".
But authorities have restricted her contact with the outside world and her whereabouts were unknown following the death of her husband, a veteran of the 1989 Tiananmen Square protests whose advocacy for democratic reform infuriated the government.
"I call on the Chinese government to release Liu Xia from house arrest and allow her to depart China, according to her wishes," US Secretary of State Rex Tillerson said.
The EU urged Beijing to let Liu Xia and his family bury him "at a place and in a manner of their choosing, and to allow them to grieve in peace".
Jared Genser, a US lawyer who represented Liu, said all contact with Liu Xia had been cut off in the past 48 hours.
"I am deeply worried about what's happening with her right now," Genser told CNN, adding that it would be hard for the government to justify continuing to hold her without charges.
"The world really needs to rally and mobilise to make sure she can go wherever she wants and that she can bury her husband wherever she wants," he said.
China rejects criticism
Foreign ministry spokesman Geng Shuang rejected the criticism of China's handling of Liu's death, adding that doctors made "all-out" efforts to treat him.
"China is a country under the rule of law. The handling of Liu Xiaobo's case belongs to China's internal affairs, and foreign countries are in no position to make improper remarks," Geng was quoted as saying by the official Xinhua news agency.
The remarks came after British Foreign Secretary Boris Johnson hit out at China for preventing Liu from travelling overseas for treatment while Germany voiced regret that Beijing ignored its offer to host Liu Xiaobo.
Liu was jailed in 2008 after co-writing a petition calling for democratic reforms and was sentenced to 11 years in prison for "subversion" a year later.
The Chinese political prisoner was represented by an empty chair at his Nobel prize ceremony in Oslo in 2010.
The Nobel Committee said Thursday the government "bears a heavy responsibility for his premature death".
But some of the global reaction to his death was muted, highlighting China's emergence as an economic and diplomatic superpower on the world stage.
US President Donald Trump and his French counterpart Emmanuel Macron offered praise for Xi at a press conference Thursday in Paris, avoiding comment on Liu's death.
UN Secretary-General Antonio Guterres said he was "deeply saddened" but refrained from criticising China for refusing to allow the Nobel laureate to receive treatment abroad.
In a sign of China's growing confidence, the state-controlled Global Times newspaper said in an English-language editorial that "the West has bestowed upon Liu a halo, which will not linger".
The government strived over the years to erase any memory of Liu and a search for his death turned up nothing on Baidu, China's Google-like search engine.
Chen Guangcheng, one of China's best-known activists who fled to the US in 2012, told AFP Liu was "deliberately killed" by the country's rulers, urging the international community to maintain pressure on Beijing.
Chen alleged that China had refused to allow Liu to travel abroad for treatment because they "were likely to discover what was really wrong with him and would probably reveal that they had been harming him with medication or some such things".
Liu's Chinese doctors had said he was not healthy enough to travel abroad, a position contradicted by US and German medical experts invited by the hospital to examine him last weekend.
But Liu Yunpeng, the hospital's head of internal medicine, said Thursday that the writer's condition abruptly deteriorated after the foreign doctors requested an assessment to determine if he could travel.
"The danger (of travelling) was extremely great," doctor Liu said.