Updated at 1500hrs:
Brent oil fell below $100 a barrel on Friday for the first time since October 2011 as weak manufacturing activity data from China and the euro-zone debt crisis raised concern of a slowdown in fuel demand.
Brent crude traded as low as $99.60, the lowest since October 4.
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Oil dived to fresh multi-month lows today, driven by weak data in top global crude consumer the United States, and as the dollar rallied on worries of a possible Spanish bailout, dealers said.
In late afternoon trade, Brent North Sea crude for July slumped to $101.27 per barrel, which was the lowest level since October 5.
New York's main contract, West Texas Intermediate crude for delivery in July, tumbled to $85.86 a barrel, which was last seen on October 24.
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"Crude oil prices plunged on Thursday, tracking sharp losses in Wall Street, while a surprisingly large build in crude oil stocks showed serious concerns about the lack of the US oil demand," said Sucden analyst Myrto Sokou.
"In addition, the fairly weak US economic data weighed further on market sentiment and limited risk appetite."
Wall Street slid today as fresh jobs and growth data painted a dull picture of momentum in the economy, while worries over Spain's teetering bank Bankia clouded the markets.
The US government's Department of Energy (DoE) announced that American crude inventories rallied by 2.2 million barrels in the week ending May 25.
That beat market expectations for a gain of just 1,00,000 barrels, according to analysts polled by Dow Jones Newswires, and indicated weaker-than-expected demand.
In more grim news, the US government cut its estimate for first-quarter economic growth to 1.9% from 2.2%, sparking questions over the strength of the recovery.