JOIN USCrowds swarm Shanghai for World Expo 2010
Tens of thousands of people were queuing before the gates opened, many arriving more than one hour early.
The organisers aim to attract an average of around 400,000 visitors daily, or 70 million by the time the Expo closes Oct 31.
More than 20 heads of state and government attended Friday's spectacular opening of the Expo, which featured fireworks and music and dance performances.
In his opening remarks before the ceremonies began, Chinese President Hu Jintao promised a "great and unforgettable" event.
European Commission President Jose Manuel Barroso opened the EU pavilion at the Expo on Saturday, calling it "a sign for the special relationship between the EU and China."
"The EU is extremely committed to the special relationship with China ... for better friendship, respect and understanding between Europe and China," said Barroso, who was scheduled to meet Hu later on Saturday.
Around 200 countries and 50 organisations have built pavilions on the 5-sq-km Expo site straddling Shanghai's Huangpu River. Visitors also have to queue to tour the individual pavilions.
This year's expo is focused on the challenges of urbanization under the slogan "Better City, Better Life."
Shanghai's 18 million inhabitants put it among the world's 10 largest cities.
With pavilions costing tens of millions of dollars, exhibitors seek to gain returns by encouraging tourism and other investment.
Analysts estimate that setting up the Shanghai event cost some $54 billion, including major infrastructure projects carried out by the city government.
The expo experienced some teething problems during trial openings last month, with long queues forming at security checkpoints and pavilions, and chaos on a nearby subway line.
The organisers said the problems had been solved in time for the official launch.
Security is tight in Shanghai, with tens of thousands of police deployed and some two million "civic volunteers" on the streets.
The Hong Kong-based China Human Rights Defenders said Shanghai authorities had sent at least 10 local rights activists to labour camps and detained or placed under house arrest dozens of others to prevent them staging protests during the Expo.
World Expositions began in the 19th century as celebrations of scientific and artistic creativity, and showcases for the world's finest industrial and cultural products.
Shanghai is the first World Expo since Hannover in 2000, while the 2005 Expo in Japan's Aichi was named an "international" event.
Italy's Milan is scheduled to host the next World Expo in 2015.
The first Expo was London's Great Exhibition of Industries of All Nations in 1851. The Shanghai Expo is the first one hosted by a developing nation.