TOKYO - Asian shares tumbled Monday after Greece's voters vehemently rejected conditions set by its international creditors, deepening doubts over their future in the 19-nation eurozone.

China's benchmark initially rebounded from last week's heavy losses after regulators and the securities industry intervened to prop up the markets, but gave up those gains as the day wore on.

Greece's debt problem has long overshadowed the market and with a European summit expected Tuesday, the implications of Sunday's "no" vote remain unclear. So the initial response to Sunday's Greek referendum was negative but not panicked.

Japan's Nikkei 225 stock index dropped 2.1 per cent to 20,212.12, while Hong Kong's Hang Seng sank 4.4 per cent to 24,916.16. South Korea's Kospi fell 2.3 per cent to 2,056.88. By late afternoon the Shanghai Composite index was up 0.5 per cent at 3,705.16.

Mainland Chinese shares are somewhat isolated from world markets thanks to capital controls limiting the scope of foreign investment. The Shanghai Composite index surged nearly 6 per cent after the market opened after the central bank pledged support for market investments; 28 companies agreed to postpone planned initial public offerings, and major brokerages pledged more than $19 billion for a fund to stabilize the free-falling markets.

The Chinese securities companies say they will continue to invest in the market as long the Shanghai Composite remains below 4,500. It closed at 3,686 on Friday.

Greek referendum results Sunday showed 61 per cent of voters opted to reject demands for added austerity measures in exchange for further bailout funding, while 39 per cent said "yes."

Economists said the markets were not expecting such an emphatic "no" vote and that could send stocks downward on Monday and propel investors toward so-called "safe havens" such as U.S. Treasuries or other government bonds that are viewed as largely protected from market turbulence.

'The result was clearly a more decisive 'no' than the polls had suggested,' said Pavel Molchanov, equity research analyst at Raymond James. "This couldn't be more bearish for equities and commodities alike."

"The 'Greferendum' has turned out to be a 'Grief-erendum' at many levels," analysts at Japan's Mizuho Bank said in a commentary.

The scope for compromise in future negotiations on a financial rescue package for Greece remains unclear.

A Greek exit from the eurozone would shake markets, but the scale of its economic impact overall would be limited by the relatively small size of its $242 billion economy - less than 2 per cent of the 19-nation eurozone - and its population of 11 million.

The European economy and European banks are in much stronger shape than they were when the debt crisis flared in 2010, said Paul Christopher, global market strategist for Wells Fargo in St. Louis, Missouri.

"We think the market reaction is likely to be sharp at first but then reverse higher in the coming weeks, as long as Eurozone policymakers respond in a proactive way," he said.

Elsewhere in the Asia-Pacific, Australia's ASX S&P 200 fell 1.1 per cent to 5,476.20 and New Zealand's benchmark also slipped 1.1 per cent to 5,776.62. Shares also were lower in Taiwan and Southeast Asia.

Wall Street looked likely to have a rough start after the Independence Day weekend, with both Dow and S&P futures down 1.0 per cent. The Dow fell 0.2 per cent on Thursday to 17,730.11 while the Standard & Poors index was little changed at 2,076.78.

The win of the "no" vote Greece increases the risks of the country falling out of the 19-country euro currency union if it must issue its own currency to alleviate a cash crunch.

"It's going to be an ugly day for the markets," predicted Kathy Lien, managing director of FX Strategy for BK Asset Management. "This means more weakness for the euro and major demand for safe haven currencies."

In Asia, the euro was trading at $1.1059, down from its Friday close of $1.1114. The dollar was little changed, slipping to 122.53 yen from 122.88 yen on Friday.

Oil prices fell, with benchmark U.S. crude tumbling $2.19 to $54.74 a barrel in electronic trading on the New York Mercantile Exchange. The contract fell 3 cents to close at $56.93 a barrel on Thursday. Brent crude, a benchmark for international oils used by many U.S. refineries, fell 73 cents to $59.59 a barrel in London.