Mining giant Glencore dropped sharply, ending down 9.7% after it reported a half-year loss of $676m (Â£431m), blaming falling oil and metal prices.
Insurer Admiral rose 3.8% as investors welcomed results showing first half pre-tax profits were up 1%.
The FTSE 100 index closed down 122.84 points at 6,403.45.
Other commodity stocks were largely lower, except for Kaz Minerals which was up 6.50% after the central bank of Kazakhstan allowed its currency to devalue.
Oil shares suffered after figures showing larger than expected US stockpiles of crude pushed the price of oil lower.
US crude fell $1.80 to $40.82 a barrel, while Brent crude dropped $1.75 to $47.06. Among the UK oil firms, shares in BP  fell 1.7% while Royal Dutch Shell dropped 2%.
China's stock markets were highly volatile on Wednesday despite government efforts to stabilise them. The Chinese market ended higher after the central bank injected more funds into the financial system for the second day in a row.
On the currency markets, the pound fell 0.1% against the dollar to $1.5646 and dropped 0.37% against the euro to â‚¬1.4148.
(Close): The FTSE 100 fell to its lowest level since January amid concerns about the outlook for commodity prices and Chinese growth.