The Expo is showcasing Chinas dazzling commercial capital but also has brought a massive security crackdown and renewed harassment of political dissidents.
Chinese President Hu Jintao and other world leaders, including French President Nicolas Sarkozy, South Korean President Lee Myung-bak and European Commission President Jose Manuel Barroso, were due to attend the grandiose opening ceremony.
The Expo opens to the public on Saturday and is expected to draw 70 million people to pavilions from almost 200 nations during its run through October.
Celebrities expected to perform at the Expo opening or otherwise attend events include actor Jackie Chan, concert pianist Lang Lang, opera star Andrea Bocelli and actress Halle Berry.
Already tight security in the city was ratcheted up Friday as authorities closed the sprawling riverside Expo site to all but a few workers, journalists, and VIPs while a helicopter patrolled overhead. Police were stationed on street corners in the area, while hotels and subway stations required all bags and packages to go through X-ray screening.
Shanghai residents began crowding into areas near the river by early afternoon, hoping to get a glimpse of the evening celebrations along a stretch of the citys Huangpu river.
While the Expo is broadly popular in Shanghai, some residents of apartments near the site said security measures were growing oppressive.
Its just not convenient to get in and out anymore, said one man, who gave only his surname, Dong. He complained that local wet markets that sold fresh meat and vegetables had been demolished and paved over for Expo parking lots.
Shanghai authorities are determined to prevent crimes or disturbances that could mar the event and have tightened their enforcement of a ban on protests or public criticism of the ruling Communist Party.
Expo organizers have also tightly vetted journalists applying to cover the events. Among outlets refused permission to attend was Hong Kongs outspoken Apple Daily newspaper, according to the regions journalists federation.
As tight as security was in Shanghai, it was low key compared to measures imposed during the Beijing Olympics, when tourist visas were refused or cancelled and the capital was cleared of migrants, months ahead of the games.