Chinese leader Xi Jinping opened one of the world's longest bridges on Tuesday, during a tour to southern China that is seen by some as an opportunity for Beijing to reaffirm its commitment to economic liberalization.

The Hong Kong-Zhuhai-Macau bridge is made up of nearly 35-km (22-mile) bridge and road sections and a 6.7 km (4-mile) tunnel, and has been dubbed the longest "bridge-cum-tunnel sea crossing" in the world.

It will link the financial hub of Hong Kong to the relatively less developed western reaches of the Pearl River Delta in Guangdong province, as well as the former Portuguese colony and gambling hub of Macau.

Xi said nothing during the inauguration of the bridge on Tuesday morning other than to declare it officially open to a burst of fireworks projected onto a screen behind him.

Hong Kong authorities have defended the bridge's HK$120 billion ($15.3 billion) price tag, saying it would consolidate Hong Kong's position as a regional aviation and logistics hub.

Xi's visit to the southern economic powerhouse of Guangdong had been shrouded in secrecy, with state media making little mention of his itinerary before he showed up for the bridge opening.

Some observers see Xi's tour to the south as highly symbolic, coming on the 40th anniversary of the beginning of China's reforms, when the country began a transformation from a centrally planned to a market-driven economy with "Chinese characteristics".

Vice premier Han Zheng said the bridge would help drive China's strategic blueprint for a "Greater Bay Area" around the Pearl River Delta modeled on other global economic dynamos like San Francisco Bay and Tokyo Bay.

"The bridge stimulates the interaction and trades between Guangdong, Hong Kong and Macau, facilitates the development of the Greater Bay Area and boost the comprehensive competitiveness of the Pearl River Delta," Han said.

Some critics, however, see the bridge as a white elephant that is part of a multi-pronged push by China to exert greater control over Hong Kong, which returned from British to Chinese rule in 1997 amid promises to preserve the city's high degree of autonomy and individual freedoms denied in mainland China.

The bridge was first proposed in the late 1980s, but it was opposed at the time by Hong Kong's British colonial government, which was wary of development that might draw the city closer to Communist China.

The bridge's opening comes at a challenging time for China as it faces pressure from a trade war with the United States, volatile financial markets, mounting public debt and a slowing economy.

Xi, who is gearing up for his second five-year term, has also come under fire for his state-centred approach to economic policy and his focus on stability, rather than proactively pushing market liberalization.

Xi said last month, however, that China was still determined to reform and wanted to work with all parties to build an open world economy.

He said on Sunday the ruling Communist Party would always support private firms' development, and any acts to weaken the private economy were wrong, according to the official Xinhua news agency.

Some Hong Kong media have dubbed Xi's trip, his second to Guangdong since taking office in 2012, as a "southern tour", echoing a high-profile 1992 visit to the special economic zones of the south by former paramount leader Deng Xiaoping.

Deng's trip helped reaffirm China's commitment to reform after the international condemnation following the 1989 killings of protesters in and around Beijing's Tiananmen Square.

A senior Western diplomat said Xi's southern tour was an opportunity for him to say something significant on economic and market reforms, and to re-affirm Deng's legacy, noting that Xi had displayed "very little evidence of that" in recent years.