Visitors, from home and abroad, have thronged to the gates of the Expo park, receiving security checks in long queues. More than 350,000 tickets have been sold or distributed for the opening day, organizers said.
Ge Ning, an executive of a sci-tech company in neighboring Jiangsu Province, was among the first to enter the park.
"I'm here to see new inventions concerning science, technology, environmental protection, and energy," he said. The United States Pavilion and the exotic pavilions of African countries are also on his list.
"I might only be able to visit five pavilions today," he said, wowing the crowds around him.
Top Chinese political advisor Jia Qinglin and International Exhibitions Bureau President Jean-Pierre Lafon activated the opening device together at a commencement ceremony.
The Expo, carrying a theme of "Better City, Better Life", reflects the crystallization of wisdom about urban construction and vision of a better future life, Jia, chairman of the National Committee of the Chinese People's Political Consultative Conference, said while addressing the ceremony.
In his speech, Jia expressed gratitude to all participants, the BIE and those who toiled for the preparation for the grand event.
In the Expo, visitors could enjoy pavilions with original design, explore scientific and technological progress, enjoy various cultural events, and take part in symposiums to fully experience the mankind's pursue for development of civilization, he said.
BIE Secretary Teneral Vicente Gonzalea Loscertales said the Shanghai World Expo would be "the most splendid and unforgettable" and would help the world better understand the present, have a glimpse into the future and strengthen cooperation between countries and organizations.
The Expo is "a celebration of human achievements with the goal of inspiring present and future generations to pursue not only the ideals, but also the actions for 'Better City, Better Life'," Loscertales said.
The six-month Expo will "not only educate, entertain and fascinate visitors, but will spark the energies to build the cities of the future on some of its foundations," he said.
Covering an area of 5.28 square kilometers along both sides of the Huangpu River, a tributary of the Yangtze River, the Expo is expected to attract 70 million visitors from home and abroad.