A lawyer linked to Ai Weiwei went missing on Thursday night and a designer from the company handling the artist's affairs was taken by police six days ago, according to supporters.

Friends have not been able to reach Liu Xiaoyuan for almost 24 hours. The rights lawyer posted a message on a microblog at 8pm on Thursday saying he was being "followed by identified people". His phone is switched off.

Last week he said he would "of course" act for Ai if requested. He spent several hours at a police station on the day Ai disappeared, although his brief detention did not appear to relate to the artist. It occurred after he requested to visit a female activist and officers reportedly berated him for tweeting about another missing lawyer.

Separately, a letter issued online on Friday said plainclothes police seized designer Liu Zhenggang, 49, at his home in Beijing on 9 April and no one had been able to reach him since. Liu worked for FAKE, the design and architecture firm that handles Ai's affairs and belongs to the artist's wife.

Police did not respond to queries about the two men.

Ai's detention has sparked an international outcry, and his case is far from alone. The last two months have seen dozens of lawyers, dissidents and activists being criminally detained and arrested or simply going missing in one of the toughest crackdowns for years. It appears to have been sparked by anonymous calls on websites overseas for "jasmine revolution" protests inspired by the Middle East uprisings.

Ai was stopped at Beijing airport on 3 April and has been incommunicado ever since. Officials have said he is under investigation for economic crimes but police have still not informed his family that he is detained.