For the first time since October 2011, oil fell below $100 a barrel on Friday as concerns of a slowdown in demand for fuel were raised by weak manufacturing data from China and the euro zone debt crisis.

In May, China's official Purchasing Managers' Index eased to 50.4 that is the weakest reading this year. The manufacturing sector of Germany contracted at the fastest pace for almost three years, and the euro fell to a 23-month low against the dollar on concerns about the Spanish banking sector.

"We've had constant worries about Greece, Spain, the euro, poor data from the U.S., and overnight the Chinese data was not positive," said Tony Machacek, an oil futures broker at Jefferies Bache.

"I think we are getting to levels where we might see a bit of stability coming back to the market."

Brent crude was down $1.80 to $100.07 by 5:58 a.m. EDT (0958 GMT) after earlier hitting a session low of $99.60.

"I don't think Friday's numbers are going to be any better. It's been a dismal week so far, and we haven't hit bottom," said Jim Ritterbusch, president at Ritterbusch & Associates and who predicted the drop in Brent below $100.

"We want a price around $100, that's what we want," Saudi Oil Minister Ali al-Naimi said on May 13. "A $100 price is great."