BEIJING, July 24 (Reuters) - Hundreds of sections of embankments along China's third-longest river have become loose, threatening the homes of millions of people after three weeks of deadly floods across the country, state media said on Tuesday.

Torrential rain has wrought havoc across large parts of China this summer, most recently in the southwest and the east, killing more than 500 people and causing billions of dollars in damage.

More rain is forecast.

The swollen Huai River has displaced about half a million residents people since the start of this month in the central province of Henan and the eastern provinces of Anhui and Jiangsu, many of whom are still unable to return home.

Tens of thousands of troops were on guard to battle any breaches along the Anhui section of the Huai, which has reported 546 places of "danger", including 46 serious ones, Xinhua news agency said.

Dozens of villages were deliberately inundated in Anhui to ease pressure at the height of the flood, which Xinhua said was "moderating". That could change with more rain forecast on Tuesday in the upper reaches of the Huai.

"People's physical and financial strength is wearing out. They tend to be less alert," Ji Bing, a top flood control official in Anhui, was quoted as saying.

In southwestern Chongqing, residents were coping with the aftermath of the worst rainstorm in more than a century. At least 42 died in floods, landslides and other disasters.

Tens of thousands of rural residents whose houses were destroyed were living in schools and tents and depended on rations, Xinhua said, adding downpours were expected to batter the area again on Tuesday.

In the northern province of Shanxi, 11 coal miners remained trapped after flash floods triggered by heavy rain submerged their pit on Sunday as rescuers were hampered by rocks and mud, Xinhua said.