The bridge connects the east and west sides of the Pearl River Delta in South China with a span of 34 miles. Chinese President Xi Jinping on Tuesday officially opened at a ceremony in the southern city of Zhuhai.
"I declare the Hong Kong-Zhuhai-Macau bridge officially open", Xi said as digital fireworks exploded on a screen behind him in an indoor ceremony, before leaving the stage immediately.
Travel time will be cut off across the delta from several hours to just 30 minutes with the opening of the world's longest sea-crossing bridge.
He said the bridge demonstrates China's engineering capabilities.
The bridge would help Hong Kong and Macau be integrated to mainland China, he added.
The transport department has already capped the number of permits for cross-border private cars at 5,000, it said.
The total price tag is unclear but some estimates run to over 100 billion yuan ($14.4 billion).
Most people will need to travel on coaches and buses. Drivers will be able to park their vehicles at the Hong Kong port and get on a shuttle bus or special hire cars after going through the immigration.
Online commenters in Hong Kong complained about the bridge's restricted access ahead of the launch. "We're now one family".
But residents in Zhuhai welcomed it.
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.
China claims credit for building the world's longest bridge, the 164.8-km-long Danyang-Kunshan Grand viaduct on the Beijing- Shanghai high speed railway. China is hopeful the opening will bind the regions together giving way to better future economic growth.
It was also timed with a wider push to mark the 40th anniversary of China's economic reforms, against the backdrop of major challenges to the economy, including the escalating US-China trade conflict, Lam said.