BEIJING (Reuters) - China's economic growth slowed slightly as expected in the third quarter as the government's efforts to rein in the property market and debt risks tempered activity in the world's second-largest economy.

The economy grew 6.8 percent in the third quarter from a year earlier, in line with the estimate produced by a Reuters poll and down from 6.9 percent in the second quarter, the National Bureau of Statistics said on Thursday.

Analysts had penciled in a gradual GDP slowdown due to an expected softening in property investment and construction as more cities try to cool surging housing prices, while a government campaign against riskier lending pushes up borrowing costs.

New construction starts measured by floor area rose 6.8 percent in January-September, slowing from a 7.6 percent rise in the first eight months of the year, the NBS said.

While policymakers' efforts to curb property market speculation and cut debt are expected to have hit growth in some parts of the world's second largest economy, activity has been supported by better-than-expected expansion in trade and bank lending.

Beijing has set a slightly more modest growth target of around 6.5 percent for 2017, theoretically offering more room for reforms after the economy grew 6.7 percent in 2016 - the weakest pace in 26 years.

GDP in the third quarter grew 1.7 percent quarter-on-quarter, compared with growth of 1.8 percent in April-June, which was revised up from initially reported 1.7 percent growth.

Analysts had expected third quarter GDP would grow 1.7 percent on a quarterly basis.

China's factory output grew 6.6 percent in September from a year earlier, beating expectations, while fixed-asset investment expanded 7.5 percent in the first nine months of the year, missing forecasts.

Retail sales rose 10.3 percent in September from a year earlier, compared with analysts' expectations for a 10.2 percent rise.

Data last week showed China's import and export growth accelerated in September, suggesting the economy is still expanding at a healthy pace. China's banks also extended more loans than expected last month, buoyed by demand from home buyers and companies.

(For graphic on China GDP click: http://fingfx.thomsonreuters.com/gfx/rngs/1/599/752/index.html)

(For graphic on China economy click: http://fingfx.thomsonreuters.com/gfx/rngs/CHINA-ECONOMY/010031D432Z/index.html)

(Reporting by Elias Glenn and Marius Zaharia; Editing by Sam Holmes)