By Gregor Stuart Hunter 

Chinese shares fell sharply early Wednesday, even as Beijing summons new measures to arrest a three-week selloff.

The Shanghai Composite is down 6.2% at 3496.99. The smaller Shenzhen Composite fell 4.1% to 1,852.98, while the ChiNext board, composed of small-cap stocks, sank 2%.

In Hong Kong, the Hang Seng Index fell 4.6% and a gauge of Chinese companies listed offshore in the city, known as H-shares, fell 6.4%.

On Wednesday, China's central bank pledged "various channels" to provide liquidity to the stock market, including to China Securities Finance Corporation, which funds margin lending by brokers. The arm of the securities regulator also will increase purchases in small-cap stocks, the regulator said.

China has put an arsenal of measures to work in recent days to stem the selloff that has wiped out roughly $2.4 trillion in value from China's equities. Over the weekend, Beijing suspended initial public offerings and made it easier for investors to borrow to buy stocks. China's brokers also vowed to buy shares until the Shanghai Composite hits the 4500 level.

Still, concerns are brewing that Beijing's increasingly desperate measures to calm markets are building bigger risks into the country's financial system, particularly a commitment from the People's Bank of China to provide unlimited liquidity support to China Securities Finance Corporation.

A huge chunk of the stocks listed on the Shanghai and Shenzhen markets remain suspended. A total of 1,544 companies in the Shanghai Composite and its Shenzhen counterpart are halted from trading today, representing 54.7% of index constituents, according to data from FactSet.

"The rescue plan could potentially increase the systemic risk down the road," analysts from Société Générale wrote in a research report. "Initially most of the stock market risk was with households, but with the rescue plan, systemically important institutions are taking up more risk when the market is still under immense downward pressure. Our biggest concern is that the progress of structural reform could suffer as the result."

Worries about China's faltering demand amid the stock slide are also driving down commodity markets, with copper hitting a six-year low on Tuesday. China is the world's top copper consumer, accounting for about 40% of global consumption. Pessimism about China, coupled with worries about a supply glut, also sent U.S. oil prices to a near three-month low Tuesday.

Brent, the global oil benchmark, rose 37 cents, or 0.7%, to $56.85 a barrel on ICE Futures Europe, after falling as low as $55.10 a barrel earlier in the session.

"Fears about the risks to financial stability and the wider economy have contributed to negative sentiment toward commodities," analysts from Capital Economics wrote in a research report. "The impact has been felt most in industrial metals, such as copper, where China is by far the most important consumer."

Meanwhile, eurozone leaders set a Sunday deadline for Greece to come up with a new set of more stringent measures to avoid defaulting on the European Central Bank and exiting the currency union. While the leaders raised the possibility of some short-term financing to help Athens make a July 20 payment, many of the policy overhauls and budget cuts demanded were overwhelmingly rejected by Greek voters in a referendum last weekend.

The Nikkei 225 Stock Average declined 1.2% in early trading while Australia's S&P/ASX 200 fell 0.6%. South Korea's Kospi Composite was flat.

Global bond markets rallied as investors sought haven assets in developed-country debt. Yields on benchmark U.S., German, and U.K. bonds hit their lowest levels in more than a month. On Tuesday, the International Monetary Fund warned of the risks of raising rates too early in the U.S., and called for the Fed to delay a raise until 2016. Concerns that the Federal Reserve would raise rates prompted a bond selloff in June, given concerns that higher rates would lower their outstanding value.

The euro also sank 0.5% against the Japanese yen as investors sought assets perceived to be safe.

Write to Gregor Stuart Hunter at gregor.hunter@wsj.com