Ask a Chinese online shopper this question: "Are you still waiting for the products you ordered during the Singles' Day promotion?"

The answer is likely to be a big fat "yes".

The online shopping promotion on Nov 11 has led to a massive delivery delay across the country as well as mounting complaints over fake discounts, substandard products and poor after-sales services.

They could trigger a credibility crisis for online shops that are preparing for a similar promotion for Thanksgiving, which falls on Thursday, said consumer rights experts.

"The complaints we're receiving these days have more than doubled, most of which are about delivery delays of goods ordered on Singles' Day," Yao Jianfang, director of the complaint department of China E-Commerce Research Center, told China Daily on Tuesday.

Yao's department helps protect online consumers' rights. It received about 40 complaints a day in the past week.

Shop owners on Taobao Mall, China's largest business-to-consumer platform, secured orders worth 3.36 billion yuan ($528 million) during Singles' Day, more than triple the same day last year.

As an unofficial Chinese festival, Singles' Day, which falls on Nov 11, is celebrated mostly by unattached young people. Online shop owners launch promotions during the festival to boost sales during the traditionally slow business season.

Zhu Chengjun, a 26-year-old white-collar worker in Shanghai, said he joined the early hour shopping rush that day and spent nearly 1,000 yuan on Taobao Mall.

But his orders were delivered later than usual - the earliest arrived after five days while the latest arrived on Monday, 10 days after the order was made.

Luan Rong, a manager of Amier online store, one of the leading lingerie brands on Taobao Mall, admitted that she has received more than 20 complaints a day since the Singles' Day sale, double a regular day.

Most complaints are about delivery delays and damaged goods, she said.

Taobao Mall did not reveal how many complaints it has received. But it said a survey had shown that 10 percent of products ordered on Nov 11 had not been delivered to consumers in five major Chinese cities including Beijing, Shanghai and Guangzhou, five days after Singles' Day. The company, however, didn't reveal any comparable figures. It usually takes only two to three days for goods ordered online to be sent to buyers in major Chinese cities.

Daniel Zhang, president of Taobao Mall, admitted that logistics has become a bottleneck.

The total number of parcels from Taobao Mall and its sister consumer-to-consumer platform Taobao.com reached a record high of 28.5 million on Nov 11, compared with 8 million on an average day, Zhang said.

"The Nov 11 figure exceeded the total amount of packages that all Chinese courier firms can deal with in one day," Zhang said.

Couriers said they have tried their best.

"We have been working on a 24-hour shift since midnight of Nov 11 and added 10 more workers to help with parcel collecting and counting. All the delivery people were on duty for the past week," said Fang Qingfeng, a manager at Shentong Express' Shaoxing branch, Zhejiang province.

The outlet dealt with more than 15,000 parcels a day between Nov 11 and Nov 17, nearly double the normal amount.

Zhang Xiaohui, a courier from Yunda Express in Shanghai, said "products sent to some suburban areas may be delayed because I received too many parcels from Taobao Mall and other online shopping platforms".

"I got 30 more parcels to deliver a day and I had to work until 9 pm to finish the day," Zhang Xiaohui said.

An industrial leader said poor logistics capacity will continue to haunt the online shopping sector.

Da Wa, secretary-general of China Express Service Association, said the demand for express services will far exceed the supply for at least five years.

Da predicted the number of domestic delivered packages will exceed 3.5 billion by the end of this year.

Ma Junsheng, director-general of State Post Bureau, said on Tuesday that the number of packages handled in China could reach 6.1 billion in 2015.

Online shopping, a sector with sales of 513.1 billion yuan or 3 percent of the total retail sales in China last year, has created half of the demand of express services and made China the world's third-largest express delivery market, trailing the United States and Japan.

China's online retail sales will surpass 3 trillion yuan in 2015, five times more than in 2010, said Xu Yu, an official with the Ministry of Industry and Information Technology.

Despite the avalanche of complaints following the Singles' Day sales boom, online shops are keen to launch similar promotions during the upcoming shopping seasons such as Thanksgiving, Christmas and the New Year's Day.

Luan said her shop will boost inventories to meet the holiday demand.

But experts urged the regulation of online shopping, a fast-growing sector where irregularities and disputes are also rising rapidly.

The Beijing Administration for Industry and Commerce has said that the total number of complaints about online shopping exceeded 1,200 in October with delays in deliveries and refunds causing the most complaints.

Liu Junhai, vice-chairman of the China Consumers' Association, called to strengthen the regulation on the sector.

"A draft plan on solving Internet retail disputes was discussed by the government to provide an improved online platform for customers and better protect them," Liu said.

Yi Shenghua, a lawyer at Beijing-based Yingke Law Firm, blamed online retailers for accepting a big amount of orders that are beyond their capability.

"The online shop owners have to be responsible for the delays and refunds of items ordered by the customers to ensure that the buyers receive well-packed high quality items in the promised period," Yi said.

Yi added that the sellers have to make sure they always have enough goods to meet the demands from customers.