Liu's close friends Gao Yu, a veteran journalist in Beijing, and Wu Yangwei, better known by his pen name Ye Du, said Liu Xia took a Finnair flight to Berlin on Tuesday morning. Wu said he spoke to Liu Xia's older brother, Liu Tong.
"Liu Xia has been kept isolated for so many years," Wu said by phone from the southern city of Guangzhou. "I hope that being in a free country will allow Liu Xia to heal her long-standing traumas and wounds."
A Chinese foreign ministry spokeswoman, Hua Chunying, said Liu left for Germany to seek "medical treatment on her own accord."
Liu Xia is an accomplished artist and poet who reluctantly followed her husband into politics two decades ago. In 2009, China sentenced Liu Xiaobo to 11 years in prison on a charge of inciting subversion after he helped write Charter 08, a manifesto calling for political and economic liberalization.
Liu was awarded the Nobel prize on Oct. 8, 2010. As soon as Liu Xia returned home from visiting her husband in prison that month, she was confined in her fifth-floor apartment in Beijing and denied access to a phone and the internet.
At first, she was optimistic her confinement would be brief, telling AP reporters at the time: "I believe they won't go on like this forever."
But the days turned into months, and then years.
Guards ate and slept outside her door, driving away well-wishers, activists, journalists and diplomats - a slow-burning ordeal worse than death, she said in a rare recording that emerged in May.
"If I can't leave, I'll die in my home," Liu Xia told her close friend Liao Yiwu, a writer who documented their phone conversation in an essay published in May.
Liu's friends said her psychological condition had steadily deteriorated, particularly since the death of her husband.
"Xiaobo is gone, and there's nothing in the world for me now," Liu tearfully told Liao. "It's easier to die than live. Using death to defy could not be any simpler for me."
Liu's release was rare good news for China's beleaguered community of activists, who have been the focus of an expansive crackdown on civil society, rights lawyers and other independent groups the administration of President Xi Jinping deems a threat to the ruling Communist Party's grip on power. The last time China let a high-profile political prisoner leave was in 2012, when blind activist Chen Guangcheng was allowed to fly to New York after escaping from house arrest and hiding for six days in the U.S. Embassy in Beijing.
Authorities are still holding Liu Xia's brother, Liu Hui, who was convicted of fraud and imprisoned in a case supporters say was in retaliation against the attention given the Nobel laureate.
"This is fantastic news, something we have all been hoping against hope for a long time," said Hu Jia, a family friend and Beijing-based activist. "But we still fear for Liu Hui, who is being kept in the country as a guarantee so that Liu Xia does not speak out abroad."
China had criticized calls by Western governments for Liu's release as interference in its domestic affairs and insisted that Liu Xia was free.
Last year, she appeared pale, gaunt and somber in images released by the authorities as she cared for Liu Xiaobo just before his death from liver cancer in a hospital under police custody. She was shown at his closely staged funeral dressed in black and wearing dark sunglasses as she clutched a photograph of her husband.
Liu Xiaobo was only the second Nobel Peace Prize winner to die in police custody, and human rights group say that shows the Communist Party's increasingly hard line. The first, Carl von Ossietzky, died of tuberculosis in Germany in 1938 while jailed for opposing Adolf Hitler's Nazi regime.
Frances Eve, a researcher for Chinese Human Rights Defenders, said Liu Xia's release was likely intended to mute criticism around the anniversary of Liu's death.
"I think the government wanted to try and save face, and make it seem as though it is a country ruled according to law when everything about her case has shown demonstrably that it is not," Eve said. "She has been an unwilling symbol of the brutality of China's treatment of human rights activists."
Shih reported from Beijing. Associated Press writer Christopher Bodeen and Yanan Wang contributed to this report.