* CSI300 -1.4 pct; SSEC -2.4 pct; HSI -1.1 pct
* Factory activity less than estimated - private survey
* Market regulator suspends a trading account of Citadel
HONG KONG, Aug 3 (Reuters) - China stocks fell on Monday with thin buying interest as a survey showed weak factory activity and some investors stayed on the sideline for a breather after mainland shares suffered their worst monthly loss in six years in July.
Factory activity in China shrank more than initially estimated in July, contracting by the most in two years, a private survey showed on Monday.
China's markets regulator has suspended a trading account of U.S.-based hedge fund Citadel LLC, the fund said on Monday, in the watchdog's first known move against a big foreign investor as it battles to prop up China's ailing stock markets.
"Buying interest was very thin, while concerns over fundamentals such as measures to deal with slowing economy are still haunting the market," said Steven Leung, a director at UOB Kay Hian in Hong Kong. "Investors lacked confidence as the latest data showed no sign of improvement."
Growth at China's big manufacturing companies unexpectedly stalled in July as demand at home and abroad weakened, an official survey showed on Saturday.
The CSI300 index fell 1.4 percent, to 3,763.42 points at the end of the morning, while the Shanghai Composite Index lost 2.4 percent to 3,574.80 points.
China CSI300 stock index futures for August fell 1.2 percent to 3,615.4, or 148.02 points below the index's current value.
The official China Securities Journal reported on Monday that the country's securities regulator will strengthen supervision of programme trading.
Analysts said investors in Hong Kong awaited major earning reports.
China Mengniu Dairy and Ping An Insurance led falls, sending the Hang Seng index down 1.1 percent to 24,372.64 points.
The Hong Kong China Enterprises Index lost 1.7 percent to 10,942.23.
The index measuring price differences between dual-listed companies in Shanghai and Hong Kong stood at 136.58.
A value above 100 indicates Shanghai shares are at a premium to those of the same company in Hong Kong, and vice versa.
There were net outflows of 1.24 billion yuan ($199.72 million) from Hong Kong to Shanghai under the Stock Connect scheme. The daily northbound quota is 13 billion yuan.
Total volume of A shares traded in Shanghai was 16.70 billion shares. Shenzhen volume was 12.09 billion shares.
Total trading volume of companies included in the HSI index was 0.7 billion shares.
China stock market graphics suite: reut.rs/1HcUe19
(includes timeline of crash, PE ratios, market caps)
A-share account openings spike: bit.ly/1wvJ9S9
Comparison of stock indexes and selected company stocks:
Chinese A-shares vs developed and emerging stocks:
link.reuters.com/rac25w ($1 = 6.2087 Chinese yuan) (Reporting by Donny Kwok; Editing by Richard Borsuk)