SHANGHAI, China — China's banking industry officially opens to full foreign competition on Monday, a landmark for the country's financial sector and a day of reckoning for the country's mostly state-owned banks.

The world's biggest banks — Citigroup Inc., HSBC Plc and Bank of America among them — have spent billions of dollars and devoted huge resources to positioning themselves for this moment.

The change, agreed to under conditions set when China joined the World Trade Organization in 2001, is the closest China, whose financial markets remain tightly regulated in many areas, has come so far to a "Big Bang" along the lines of the unprecedented 1986 overhaul of London's financial world.

But as avid as global banks may be to tap China's $4 trillion pool of household and commercial wealth and its fast-growing market for financial services, they are unlikely to win a major share of the industry anytime soon, analysts say.

"It is really difficult for foreign banks organically to grow, particularly in as large and idiosyncratic a market as China," says Michael Pettis, a professor of finance at Peking University's Guanghua School of Management.

The rules that take effect Monday will give foreign banks access to the local currency retail banking business, in theory lifting all geographic and client restrictions on operations. Previously foreign banks were allowed to offer such services, on a limited scale, only in 20 major cities.

Preparing for those changes, foreign lenders have sought multibillion-dollar stakes in local banks, expanded their branch networks and aggressively targeted wealthy clients with private, foreign currency banking services.

By the end of June, there were 71 foreign banks in mainland China with 183 branches, the most recent figures available.

In Shanghai, the Chinese mainland's main international commercial center, foreign banks held 13 percent of all banking assets by the end of 2005. But nationwide, they account for an aggregate market share of less than 2 percent.

History suggests they are unlikely to grab a significantly bigger share of the market anytime soon, despite their overall stronger asset bases and better services, Pettis says.

In countries as varied as Russia, Brazil and the United States, market opening has not resulted in an exodus of customers from local banks to foreign ones.

"It's just never happened," he says.

Nonetheless, to meet what is viewed locally as an onslaught of foreign competition, local banks have revamped their operations, with the biggest getting multibillion-dollar government bailouts to recapitalize after writing off huge amounts of bad debt.

They've also sought out international lenders as strategic investors to draw in both money and management skills, maneuvering to control risks and improve their services.

"Local banks have done a lot," says May Yan, banking analyst at Moody's Investors Service in Hong Kong. "They were under the pressure of opening up the market. Now they are better financed and run on a more commercial basis."

Foreign banks are likely to speed up their expansion following the latest reforms, she says.

But plenty of hurdles remain.

The biggest may just be size. Although huge on a global scale, international banks are dwarfed by their Chinese rivals.

The biggest state-run commercial bank, Industrial and Commercial Bank of China, has 18,000 branches and deposits totaling some $700 billion. The other big Chinese banks, although smaller, operate on a similarly grand scale.