SHANGHAI stocks tumbled by their most in a month after a widely watched China manufacturing index unexpectedly contracted and Tokyo shares, plunging 7.3 percent, led a broad Asian decline.

The Shanghai Composite Index slid 1.16 percent to a one-week low of 2,275.67 after the HSBC Flash China Manufacturing Purchasing Managers' Index, a preliminary reading of factory activity, slipped to a seven-month low of 49.6 in May from 50.4 in April. A reading of below 50 indicates contraction in activity and, by extension, slower economic growth.

Analysts said market sentiment was hurt when Japan's Nikkei 225 Stock Average dropped by its most since the earthquake and tsunami two years ago and signals from the US Federal Reserve on the duration of the central bank's asset-purchase program turned more mixed.

"Weaker-than-expected manufacturing activity in China spurred a risk-off sentiment across Asia and hurt the domestic stock market," said Zhou Jianwen, a researcher with Bozhong Securities Investment Consultancy. "Manufacturing-related stocks may come under great pressure."

Anhui Conch Cement Co dropped 3.2 percent to 17.31 yuan (US$2.82). Shaanxi Qinling Cement (Group) Co shed 3.6 percent to 5.84 yuan.

Jiangxi Copper Co, China's largest producer of the metal, lost 2.8 percent after media reports said some banks have halted financing for copper imports.