BEIJING — The ailing widow of Liu Xiaobo, the Chinese democracy advocate and Nobel Peace Prize laureate who died of cancer last year under police guard, left China for Europe on Tuesday after a high-level diplomatic campaign by the German government.

Diplomats said Liu Xia, Mr. Liu’s widow, flew to Helsinki, Finland, on Finnair. Her final destination is expected to be Germany.

Ms. Liu’s brother, Liu Hui, posted a message soon after the flight took off saying she had left for Europe “to start her new life.” He thanked all the supporters who had helped win her release from years of house arrest and strict police supervision.

Chancellor Angela Merkel of Germany raised Ms. Liu’s case directly with China’s leader, Xi Jinping, during a visit to Beijing earlier this year, a gesture that underscored her opposition to China’s treatment of Ms. Liu and her husband, European diplomats said.