Visitors endure the heat as they patiently wait in queues to enter the China Pavilion in the Expo Garden on August, 1, 2010. The temperature that day rose to 34 C.[Xinhua]

Security has remained a priority of Expo organizers, as the six-month fair passed its 100-day milestone on Sunday.

In a recent interview with local media, the city's Party chief, Yu Zhengsheng, said maintaining order and security would remain key issues for Expo organizers until midnight on October 31, when the event draws to a close.

"Our pre-emptive measures, management rules and vulnerable spots are publicly known. Even those who have the intention of making trouble are aware of them, so we can never ease up on security," he said.

Shanghai Expo 2010, which opened on May 1, is a further sign of China's growing global importance following the 2008 Beijing Olympics.

The event is currently on course to become one of the largest gatherings in history, with 189 participating countries. Organizers also expect it to surpass cultural milestones set by previous World Expos, such as when the Ferris wheel was introduced at the 1893 Columbia Expo.

More than 38.6 million people have visited the Expo as of Tuesday, most of them from the mainland. Organizers anticipate that total attendance will exceed the previously estimated 70 million despite fewer visitors in the early days of the event.

During the interview, Yu repeated that the 70 million is an estimate and not a target. He also pledged that the city would not take any forced measures to boost attendance.

"The public endures a hard time when they visit the Expo," Yu said, citing the difficulties of finding food and transportation to the Expo Garden.

He said the attendance is relative to the success of the fair, but not a defining factor. "We can call it a success if we have 50 or 60 million visitors."

The Expo Garden has a maximum capacity of 600,000 visitors per day, Yu said, adding that organizers expect a large influx of visitors in September and October, when temperatures will have cooled off.

On the post-Expo fate of foreign pavilions, especially those that cost tens of millions of dollars, Yu said the situation was still under discussion.

According to the rules of the Bureau of International Expositions, the governing body of World Expos, all foreign pavilions are required to be dismantled after the Expo and the site redeveloped.