Beijing, Oct 23 (IANS): Chinese President Xi Jinping on Tuesday officially opened the world's longest sea-crossing bridge that connects mainland China with Hong Kong and Macau, during a ceremony in the city of Zhuhai.

The $20 billion mega-project includes with a 55 km road bridge that has been in the works for almost nine years, CNN reported.

It's a key element of China's plan for a Greater Bay Area covering 56,500 sq.km of southern China, and encompassing 11 cities, including Hong Kong and Macau, that are home to a combined 68 million people.

Proponents of the bridge have said that it will significantly cut journey times between the cities, enabling commuters and tourists to easily move around the region.

However, private car owners in Hong Kong will not be able to cross the bridge without a special permit.

Most drivers will have to park at the Hong Kong port, switching to a shuttle bus or special hire cars once they are through immigration.

Shuttle buses will cost between $8 to $10 for a single trip depending on the time of day.

Built to withstand a magnitude 8 earthquake, a super typhoon and strikes by super-sized cargo vessels, the bridge incorporates 400,000 tonnes of steel -- 4.5 times the amount in San Francisco's Golden Gate Bridge, CNN said.

It also includes a 6.7 km submerged tunnel to help it avoid the busy shipping paths over the Pearl River Delta.

The tunnel runs between two artificial islands, each measuring 100,000 sq.metres and situated in relatively shallow waters.