China Construction Bank's online mall saw a 67 percent surge in transactions last year, while Agricultural Bank of China had a 57 percent gain, faster than the 47 percent growth at Alibaba's Tmall and Taobao sites.

Bank of Communications Co's e-commerce platform sells merchandise and also allows customers to book air tickets and hotels. China Merchants Bank Co's online shop offers a 30 percent discount on Prada bags, and zero interest and free shipping on purchases made on installments through its credit cards.

"It's not going to become a profit engine," said He from BCG. "But it offers invaluable support to their core business by extending their product line, increasing customer stickiness, getting more data and adapting to the digital era."

ICBC's mall lets customers also purchase wealth management products, gold bars, luxury cars, travel packages and even apartments. Developers like China Vanke Co and Evergrande Real Estate Group Co are selling properties on the Beijing-based lender's site. Customers can get an almost 10 percent discount on some homes and lower rates on mortgages from the bank. More than 1,000 apartments worth 1.1 billion yuan have been sold.

"China is undergoing an experimental and revolutionary phase right now," said Joe Ngai, Hong Kong-based head of McKinsey & Co banking practice.

"It's fair game in China, where everyone is trying to improve on the consumer experience. This is a great thing for the consumer."

Revenue from online retailing is expected to reach 4 trillion yuan this year, according to the China e-Business Research Center. By the end of the year, more than 850 million Chinese are expected to be online, according to the Ministry of Industry and Information Technology.

Retail banking revenues will double to 3.5 trillion yuan by 2020, BCG estimates show.

Spokesmen for ICBC and China Construction Bank declined to comment on the competition, and Alibaba and Tencent did not reply to e-mails. As of Wednesday, ICBC had a market value of $302 billion versus $210 billion for Alibaba.

"The future of finance, including banking, no longer hinges on brick-and-mortar operations, but integration with the web, the mobile and serving customers anywhere 24/7," said Chen Xingyu, a Shanghai-based analyst at Phillip Securities Research. "Chinese banks seem to already have a leg up over many global rivals."