* Rebar drops as much as 7.6 pct, widening losses
* Clocks up 16 pct fall in May, after 21 pct rally in April
* Iron ore in Dalian down more than 4 pct at close
* Steel supply seen rising while demand has weakened (Updates prices)
By Manolo Serapio Jr
MANILA, May 10 (Reuters) - Chinese rebar steel futures posted their biggest intraday fall in seven years on Tuesday, as worries about demand in the world's second-biggest economy extended a rout that has spilled over into other industrial raw materials.
Rebar, or reinforced steel, fell 7.6 percent at one point, while iron ore lost as much as 6 percent. Retail buyers rushed into China's commodities futures in April, hoping a stabilising economy would boost demand for raw materials, lifting prices and volumes of everything from steel to cotton.
China's main commodity exchanges in Dalian, Shanghai and Zhengzhou have since imposed measures to rein in speculative activity including higher trading fees. big market moves and the response of authorities have raised concerns about the risk of contagion for global markets, particularly after last year's Chinese stock boom and bust
The exchanges' measures along with an increase in steel supply, particularly of rebar used in construction, had helped fuel a big retreat in futures prices, said Qing Dong Qing, analyst at Shanghai Zhong Cai Futures.
"The short positions of hedge funds and futures investment funds have also suppressed ferrous metals," said Qing.
The most-traded rebar on the Shanghai Futures Exchange SRBcv1 fell as much as 7.6 percent to 2,085 yuan ($320) a tonne, its weakest since March 18 and the biggest percentage drop since March 30, 2009. It closed down 4.9 percent at 2,147 yuan.
Rebar, which led the mid-April rally that unnerved global investors, has dropped nearly 16 percent this month after gaining a record 21 percent in April.
"Buying interest has shrunk so those steel traders who have bought cargo for speculation must now feel the pain with actual demand just so-so," said a Shanghai-based trader.
China's crude steel output hit a record high of 70.65 million tonnes in March and analysts and traders expect production to have remained strong in April as a rally in steel prices spurred mills to lift output. The April data will be released later this month.
The weakness in steel has spilled over to raw materials used to make it. On the Dalian Commodity Exchange, the most-active iron ore DCIOcv1 contract ended down 4.5 percent at 385.50 yuan a tonne after hitting a session low of 379.50 yuan, the lowest since April 11. It fell as much as 6 percent earlier in the session.
Dalian coke DCJcv1 fell 3.4 percent to 936 yuan a tonne.
Other commodities traded on Chinese platforms also dropped, with Shanghai nickel SNIcv1 down nearly 2 percent, tin SSNcv1 off 2.4 percent and aluminium SAFcv1 down 2.5 percent.
($1 = 6.5178 Chinese yuan)