* All police leave cancelled in Beijing for Party Congress
* Bejing Communist Party Chief says "we must hold the line"
* Security tightened at ports along the Yangtze River
* Foreigners won't be able to visit Tibet in 11-day period
* Migrants in Beijing targeted; metal materials market closed
By Christian Shepherd and Ben Blanchard
BEIJING, Sept 28 (Reuters) - China is tightening security for next month's twice-a-decade Communist Party Congress, cancelling police leave in Beijing, limiting tourism to Tibet, and clamping down on the spread of political rumours.
High-level meetings in China are typically accompanied by a security crackdown - as well as uncharacteristically smog-free blue skies - with the stability-obsessed party not wanting to run the risk that anyone or anything offers a distraction.
Thousands of policemen from other provinces have been sent to the Chinese capital to reinforce, a source with direct knowledge of the matter told Reuters. A second source, with ties to the country's security forces and citing conversations with senior police officers, said all police leave in Beijing had been cancelled starting from early September.
Beijing Communist Party chief Cai Qi on Wednesday asked the city for "120 percent" effort to ensure safety for the congress, the official Beijing Daily said.
"We must hold the line for social control, eliminate all destabilising factors, hold the line for cyber security and resolutely crack down on political rumours and harmful news," Cai said.
Beijing's largest market for metal building materials will be shuttered from Friday and is being relocated to Hebei province taking with it more than 10,000 industry workers and stall owners, according to the Beijing Daily.
The market's unceasing growth had created a "messy environment" where migrants congregate, the paper said, without mentioning the crucial party gathering.
Many of the tightened security measures target migrants, with ID checks at metro stations and patrols outside government ministries to ensure any petitioners from out of town are rounded up immediately should they attempt to make a scene.
Some 2,000 delegates will converge on Beijing for the Congress, staying at hotels across the city, and security will only get tighter as its opening nears, meaning any protests will be quickly shut down.
China's ongoing clampdown on cyberspace has seen WhatsApp, the messaging service run by Facebook, periodically unavailable in the past few weeks, while certain gifs using images of President Xi Jinping cannot be posted to group chats on messenger app WeChat.
China's Internet regulator did not respond to requests for comment on what was happening to WhatsApp and WeChat.
New limits have been placed on discussion in private group chats. Rules released at the beginning of September to make companies and group owners accountable for breaches of content rules.
China's cyber watchdog on Monday also imposed the largest possible fines of 100,000 yuan ($15,110) on tech giants Tencent Holdings Ltd, Baidu Inc and Weibo Corp for failing to censor online content.
The measures are far from restricted to just keeping Beijing secure. Measures have also been introduced in strategically sensitive places in other parts of China.
Security has been heightened at ports along the Yangtze River, with the Maritime Safety Administration saying last week that it was considering curbs on loading and unloading hazardous or flammable chemicals from Oct 11 to Oct 28.
Travel for foreign tourists to Tibet is also being restricted, some travel agents said. Tibet is always sensitive for Beijing given the opposition to Chinese rule that exists within the Tibetan community, and even in normal times foreigners need permission to go there.
An official at China International Travel Service Ltd said the government had told it that no foreigners were allowed to visit Tibet from Oct. 18-28 because of the Congress. Three other travel agencies confirmed a ban would be lifted in late October.
Organisers of a discussion on politics in the Middle East cancelled the event in Beijing because they were worried about potential pressure from the authorities, a source with direct knowledge of the matter told Reuters.
"The word 'democracy' was only used once in the adverts, not in a sensitive way, but still they thought it was best to cancel," the source said, declining to be named.
Feng Xiaogang, a famous Chinese film director known for work that brushes against sensitive issues and the limits of censorship, announced last week that the Sept 30 release of "Youth", his latest film, had to be delayed.
Feng gave no explanation.
However, a source with ties to the censors told Reuters that the authorities had considered it risky to screen the film before the congress, as it is partially set during China's 1979 war with Vietnam - a touchy subject. The fear was that it could spark debate about the morality and necessity of the conflict.
In and around Beijing, efforts have also been ramped up to ensure the skies remain blue over the Congress and not polluted by the city's notorious smog.
The city of Handan, near Beijing, has ordered steel mills to halve output a month earlier than the usual mid-November cutbacks aimed at curbing air pollution, according to media reports.
Rounds of inspections by the authorities to check up on health and safety or environmental requirements have left small businesses closed for days, weeks or months, owners and employees say.
Factories on the outskirts of Beijing, even those that make luxury goods, have been closed for days while inspections are carried out, according to one employee who declined to be named.
Some low-end, fast food-type restaurants in Beijing have also been ordered to close, ostensibly to prevent fire risks and to limit pollution, residents say.
"There's so many inspections at the moment," said a pancake seller who gave her family name as Liu. "What am I supposed to do if they make me shut?" (Reporting by Ben Blanchard and Christian Shepherd; Additional reporting by Cate Cadell, Michael Martina, Benjamin Kang Lim and Beijing newsroom; Editing by Tony Munroe and Martin Howell)