Liu Xia was on a Finnair flight bound for Helsinki that left Beijing at around 11 a.m. (0300 GMT), Ye Du, a writer and friend, told Reuters. "Liu Xia might not be able to speak much for fear of her brother's safety", he said.
Efforts to secure Liu Xia's departure came amid growing concern over her health and state of mind, after Liao Yiwu, a dissident and friend living in Germany, released details of a telephone conversation in April in which an anguished Liu Xia said she was losing hope of leaving.
Liu's release comes as Chinese Premier Li Keqiang is visiting Germany, a country that in May said it would welcome the widow after a recording was released of her crying in desperation and indicating she has given up hope of being able to leave China.
Her departure, following a year of pressure on the government from activists and worldwide human rights organizations, comes at the end of a visit to Germany by Chinese Premier Li Keqiang.More news: Mother is sole survivor of crash that killed husband, 4 daughters
Hua Chunying, a Chinese foreign ministry spokeswoman, said that Liu's travel to Germany for medical treatment was "of her own free will". Ye said he was informed of Liu's departure by her older brother. "May the rest of her life be peaceful and happy".
Liu Xiaobo died aged 61 in hospital on July 13 past year.
Chinese authorities put Liu under house arrest in 2010, days after the Norwegian Nobel Committee awarded her imprisoned husband, Liu Xiaobo, the Peace Prize, infuriating the Chinese government.
Following her husband's death in July 2017, Ms Liu's whereabouts were unknown for six months, although the Chinese government claimed she was "free". The post was verified by two of Liu Xia's close friends.
Merkel has spoken out frequently on Chinese human rights abuses and is believed to have pushed for Liu's release during her May visit to Beijing, where she met the wives of detained human rights lawyers.More news: Government Will Miss Deadline To Reunify Young Children With Parents
Memorial events to commemorate the one-year anniversary of Liu Xiaobo's death on Friday are planned in cities around the world including Hong Kong, Taipei and Berlin.
China sentenced Liu Xiaobo in December 2009 to 11 years imprisonment on charges of inciting subversion of state power after he helped write a manifesto calling for political and economic liberalization.
Western diplomats, however, disagree.
Liu Xia's departure from China was "wonderful news" but harassment of her family remained a risk to her freedom to criticize China, Amnesty International's China researcher Patrick Poon said.
Ms Liu, a painter and poet, was arrested and had her mobile phone deactivated when she visited her husband during his fourth prison term.More news: Is Modric the best midfielder in the world?
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