A trademark spat between Chinese e-commerce giant Alibaba Group Holding Ltd and rival JD.com flared into public view after JD published an Alibaba letter urging publishers to be careful about advertising in promotions for China's annual "Singles' Day" spree, the world's largest online shopping day.

In the letter, dated Oct. 16 and published on Thursday on a JD social media account, Alibaba's Tmall.com marketplace warned Chinese publishers against running ads with the "Double Eleven" motif that are not official Alibaba promotions. The slogan has become a well known reference to Nov. 11, when Singles' Day takes place each year, encouraging unattached consumers to buy goods as gifts for themselves.

Tmall said in the letter that "Double Eleven" is a registered trademark. According to research by Reuters, in 2013, Alibaba registered at least six trademarks associated with "Double Eleven" with the State Administration of Industry and Commerce.

The letter highlights fierce competition among e-commerce firms in China over their marketing efforts around Nov. 11, a day that has proved an online shopping bonanza. Total sales processed on Singles' Day last year approached $8 billion.

In the letter, which was published on a JD Weibo account, Tmall also condemned some of Alibaba's e-commerce rivals, without identifying names, and said media outlets would be liable for breach of trademark if they published any ads that infringed on Alibaba's rights.

"We express our extreme indignation and condemn some e-commerce companies for their demeaning activities," Alibaba's Tmall said in the letter, without disclosing specific examples of activities to which it objected. Tmall said publishers would bear joint liability for any breach of China's advertising law.

Officials at Alibaba and JD weren't immediately available for comment on Thursday.

JD said on Weibo that Alibaba's warning was counter to "the open spirit of the internet and the principles of fair competition, and said the move would limit consumer choice and damage consumer interests.

For its part, Alibaba's letter amounted to a muscular effort to protect its lead on a hugely important day of sales.

Last year on Nov. 11, Alibaba's payment system alone processed $5.8 billion worth of transactions - more than the amount sold in the United States on Black Monday and Cyber Friday - with almost a third of China's population visiting Alibaba websites during the 24-hour period last year.

JD, a distant second to Alibaba in Chinese e-commerce, posted sales of $1.6 billion during its own Singles' Day promotion last year, which extended over 12 days.

Other internet merchants, including Amazon.com Inc, have already been rolling out promotions for the November shopping event. Amazon China announced on Wednesday that the company will focus on international brands during this year's Singles' Day event, allowing local customers to buy items and get direct shipments from the firm's international websites.