BEIJING — On Wednesday, he posted an appeal to change China’s Constitution, suggesting, among other things, that there might be a more democratic way to elect the country’s leader. By Friday morning, he was in police custody.

Yu Wensheng, one of China’s most prominent lawyers and political activists, was detained as he left his apartment building in Beijing to walk his 13-year-old son to school, according to his wife, Xu Yan, and two associates.

Mr. Yu, who has defied previous arrests and warnings for his outspoken appeals for political change, had already appeared to be on a new collision course with the authorities. This week, he learned that his license to practice law had been suspended and that he would not receive a passport to travel abroad because he was a security risk — all of which he daringly disclosed on social media.

Mr. Yu’s latest appeal came on the eve of meetings this week by Communist Party leaders in Beijing where changes to the Constitution were on the agenda. The changes, discussed behind closed doors in a meeting that ended Friday, could further consolidate the power of President Xi Jinping, who after the party’s major congress in October has as much political authority as any Chinese leader since Mao Zedong.