The pro-democracy Chinese dissident died on Thursday while serving an 11-year prison sentence.
Liu Xia, 56, was with Liu Xiaobo when he died of cancer at a hospital in northeastern China earlier this month. He was serving an 11-year sentence on subversion charges at the time of his death.
Liu Xia’s current location is unknown and her Beijing apartment remains tightly guarded.
Activists feared she may have been forcibly disappeared.
“Liu Xia did not commit any offence,” Patrick Poon, Amnesty International’s China researcher, told Al Jazeera.
“It’s illegal for the Chinese government to restrict her freedom, and we urge the authorities to lift all restrictions on Liu Xia and let her go anywhere she wishes.”
Amnesty’s petition urges the Chinese government “to end the illegal house arrest and surveillance of Liu Xia, stop her harassment and allow her to travel freely”.
Liu Xiaobo, a prominent dissident since the 1989 Tiananmen Square protests, leaves behind a powerful legacy that inspires others to continue the struggle for human rights in China, Amnesty said.
“Our greatest tribute to him will be to ensure that Liu Xia is free to do the same,” the rights group added.
Liu Xia, a poet who stayed out of politics, has been under effective house arrest since her husband won his Nobel in 2010.
She was never charged with an offence, but was kept guarded and largely isolated for more than seven years in the apartment she once shared with her husband.
She was last seen in an official photo taken on July 15, in which she and a few relatives lowered an urn containing Liu Xiaobo’s ashes into the Pacific Ocean near Dalian, a city in northeast China.
Only “sustained international pressure can help” Liu Xia, Poon said.
Human Rights Watch said the widow’s friends and relatives in Beijing have not been able to reach her directly.
A German embassy source in Beijing told the Reuters news agency on Friday that Germany was deeply concerned about China’s “apparent unwillingness to discuss” lifting restrictions on Liu Xia.
“A number of ambassadors have repeatedly asked for a meeting with Chinese security organs – so far to no avail,” said the source, speaking on the condition of anonymity.
The United Nations, United States, European Union and other foreign governments and groups have urged China to lift restrictions on Liu Xia’s movements.
But China has lambasted calls for her release, saying it was an internal matter.
Meanwhile, Liu Xiaobo’s supporters have also been prevented from holding commemorative activities.
At least six people were detained in the Chinese city of Jiangmen after they held a memorial for him on July 19, according to the Hong Kong Free Press.
In the days following his death, authorities in mainland China rigorously censored references to Liu Xiaobo on China’s internet, with reports that social media posts containing candle emojis and the letters “RIP” were censored for violating “relevant laws and regulations”.