The wife of a prominent Chinese human rights lawyer says her husband was taken into police custody while taking their son to school.

Yu Wensheng's detention Friday took place a day after he posted a letter online calling on the ruling Communist Party to reform the Chinese constitution and allow open presidential elections.

A police officer who would only give his surname, Chen, at the Xingucheng neighborhood police station said he had "no idea" about the case and that even if there were a case involving Yu, the officer would not be allowed to comment.

Yu's wife said police haven't told her why Yu was taken into custody or where he is being held. Yu was previously detained by police in 2014 and 2015.