China's key Shanghai Composite widened losses in the afternoon session to end down 6.5 percent at a near one-week low, marking its biggest one-day loss since January 19 and breaking an eight-session winning streak. The CSI 300 index of the largest listed companies in Shanghai and Shenzhen tumbled 6.7 percent, while the start-up board ChiNext sank 5.4 percent. News that more Chinese brokerages are tightening margin lending rules seem to be the main cause of concern among retail investors, experts say. According to IG market strategist Bernard Aw, Guosen Securities increased the margin requirement for 908 counters while Southwest Securities reduced the amount of margin financing that traders can receive using collateral. Separately, the Shanghai Securities News also reported that regulators have recently urged banks to submit data regarding money flows into the stock market, according to Reuters. Financial and property heavyweights saw steep declines; China Construction Bank (Shanghai Stock Exchange: 1939-SZ) and Bank of China (Shanghai Stock Exchange: 1988-SZ) sank more than 5 percent each, while Shanghai Shimao and Gemdale lost 9.5 and 8.9 percent, respectively.