SEOUL, South Korea — The police said on Thursday that they were searching for Park Won-soon, the mayor of Seoul, the capital of South Korea, after his daughter reported him missing.

       Mr. Park, 64, canceled his official schedule for Thursday and called in sick to City Hall. His daughter told the police that he had left home after leaving a cryptic, “will-like message,” according to Yonhap, the national news agency, which cited an anonymous police source.

       When Mr. Park had not returned home after five hours, his daughter called the police.

       The mayor of Seoul, a city of 10 million, is largely considered the second most powerful elected official in South Korea after the president. Mr. Park, who is serving his third term, has often been named as a possible candidate to replace President Moon Jae-in, whose single five-year term is set to end in 2022.

       Mr. Park has been the mayor of Seoul since 2011. His latest term is scheduled to end in 2022.

       Before becoming mayor, Mr. Park was a prominent human rights attorney who founded the country’s most influential civil rights group.

       As a lawyer, he won several major cases, including South Korea’s first sexual harassment conviction. He also campaigned for the rights of so-called comfort women, Korean sex slaves who were lured or forced to work in brothels for the Japanese army during World War II.

       A tireless critic of inequality, he was a vocal antagonist of former President Park Geun-hye, and supported huge rallies against her in central Seoul that led to her impeachment and ouster on corruption charges in 2017.