China's eighth-largest property developer by sales said late on Wednesday the Putuo Branch of the Shanghai Public Security Bureau informed it that Chairman and Executive Director Wang Zhenhua had been detained for "personal reasons".

Future Land's Hong Kong-listed shares fell as much as 18.5% in early Thursday trade, hitting their lowest since Feb. 15.

Property management unit S-Enjoy Service Group Co Ltd was down 16.2%, while subsidiary Seazen Holdings Co Ltd fell the maximum-allowed 10% in Shanghai.

Wang's detention was not related to company operations and business remained normal, Future Land said in Wednesday's filing to the Hong Kong stock exchange. It gave no further details.

Putuo police declined to elaborate when contacted by Reuters.

Wang controls about 70% of Shanghai-based Future Land. His son, 31-year-old non-executive director Wang Xiaosong, has been appointed chairman of the board with immediate effect, the firm said.

Ahead of the announcement on Wednesday, rumours of the incident sent Future Land stock closing down 23.9%, its biggest daily percentage decline since listing in November 2012. That left it with a market value of HK$47.4 billion ($6.09 billion).

S-Enjoy, formerly Xinchengyue Holdings Ltd, in a separate Wednesday statement said Wang Zhenhua, who is also a non-executive director at the firm, had been detained and that operations remained normal.

($1 = 7.7843 Hong Kong dollars)

(Reporting by Donny Kwok and Clare Jim; Additional reporting by Cheng Leng in BEIJING; Editing by Anne Marie Roantree and Christopher Cushing)