China has opened the world's longest sea-crossing bridge linking Hong Kong to the mainland, a feat of engineering carrying immense economic and political significance.

Key points: The "bridge of death" cost $20 billion

The "bridge of death" cost $20 billion An underwater tunnel is connected by two artificial islands

An underwater tunnel is connected by two artificial islands It carries major political significance for President Xi Jinping

Chinese President Xi Jinping presided over a ceremony in Zhuhai to open the 55-kilometre-long bridge, which links the city to the semi-autonomous regions of Hong Kong and Macau.

Digital fireworks exploded on a screen behind him as leaders of the three coastal cities looked on.

The $20-billion bridge took almost a decade to build while incurring major delays and cost overruns.

It was known as the "bridge of death" among some local media outlets, with officials saying at least 18 workers had died during construction, according to the BBC.

Xi Jinping applauds on stage after the official opening of the bridge. ( AP: Andy Wong )

The bridge includes an undersea tunnel connected by two artificial islands, which allows ships to pass through the Pearl River delta — the heart of China's crucial manufacturing sector.

Its opening will cut travel time across the delta from several hours to just 30 minutes, something China hopes will bind the region together as a major driver of future economic growth.

Traffic will begin flowing on Wednesday, but motorists must have a permit issued under a quota system.

An aerial view of an artificial island connected to the world's longest sea bridge. ( Weibo/CCTV )

Xi's political symbol

Sorry, this video has expired The bridge has been designed to withstand earthquakes and typhoons that lash the Pearl River Delta area. ( ABC News )

The bridge forms a physical link between the mainland and Hong Kong, an Asian financial hub that was handed over from British to Chinese control in 1997 with the assurance it would maintain its own legal and economic system for 50 years.

That carries major political significance for Mr Xi's administration, which has rejected calls for political liberalisation in Hong Kong, sparking fears Beijing will clamp down further on civil liberties before the end of the "one country, two systems" arrangement in 2047.

A white dolphin jumps out of the sea in front of the Hong Kong-Zhuhai-Macau bridge off Lantau island. ( Reuters: Bobby Yip )

The bridge's opening also comes a month after the inauguration of a new high-speed rail link from Hong Kong to mainland China that runs along a different, shorter route.

That line has vastly decreased travel times but also raised concerns about Beijing's growing influence because mainland Chinese law applies within part of the line's Hong Kong terminus.

An aerial shot of a terminal leading to the world's longest bridge from China to Hong Kong. ( Weibo/CCTV )

To Claudia Mo, a Hong Kong democratic politician, the new bridge's political significance outweighs its practical usefulness.

"It's not exactly necessary, because Hong Kong is connected to mainland China in every way already, by land, by air, by sea," Ms Mo said.

"But they still need it as a political symbol or icon to remind Hong Kong people … that you are connected to the motherland, with this very grand bridge.

"It's almost like an umbilical cord."

In Zhuhai, however, sentiments revolved around economic growth and national pride.

The world's longest sea bridge lit up in Hong Kong. ( AP: Kin Cheung )

Airline pilot Liu Gang said he had been eagerly anticipating the opening of the bridge, calling it a symbol of the mainland's increasingly close ties with Hong Kong and Macau.

"It'll bring us even closer together, make us more flexible, economically and in many other ways," Mr Liu said while strolling along a walkway and shooting photos of the structure.

"We're now one family."

Luo Fengzhi, who works in real estate, cited the bridge as evidence of China's growing economic and engineering prowess.

"For Chinese people, this makes them feel proud," she said.

"I hope that every patriotic Chinese person can come and see this great feat of engineering, and I welcome foreigners to come and see for themselves as well."

Workers in China clean a pavilion with the Zhuhai-Macau-Hong Kong Bridge in the background. ( AP: Andy Wong )

AP/ABC