The Huajiang Grand Canyon Bridge towers 625 metres (2,051 feet) above a river and vast gorge in the country's rugged southern province of Guizhou, also home to the 565-metre Beipanjiang Bridge that is now the world's second highest.

Live drone footage broadcast by state media on Sunday showed vehicles traversing the immense structure, its blue support towers partially engulfed in clouds.

Crowds of onlookers including project engineers and local officials gathered on the bridge for a ceremony to mark the occasion, several expressing their pride and excitement in live interviews to state media.

"The opening of the Huajiang Grand Canyon Bridge reduces travel time between the two sides from two hours to two minutes," Zhang Yin, head of the provincial transport department, said at a press conference Wednesday.

Its opening makes "enormous improvements to regional transportation conditions and (injects) new impetus into regional economic and social development", she said.

China has invested heavily in major infrastructure projects in recent decades, a period of rapid economic growth and urbanisation in the country.

The hilly province of Guizhou in particular is crisscrossed by thousands of bridges -- which now include the world's two highest.

State news agency Xinhua said Wednesday that nearly half of the world's 100 highest bridges are located in the province.

The Huajiang Grand Canyon Bridge took more than three years to complete, Xinhua reported. Its 1,420-metre main span makes it the "world's largest-span bridge built in a mountainous area", it added.

Apart from the world's highest bridge, the tallest -- measured in terms of the height of its own structure, rather than the distance to the ground -- remains France's Millau viaduct at 343 metres.