Shanghai's top official on Thursday urged Expo workers to step up security as the event moves into its final 30 days.

Shanghai Party chief Yu Zhengsheng expressed his concern that security staff were in danger of becoming lax as the six-month fair nears the end and alerted them to potential security hazards.

"As times go on, our people could become complacent, which is our greatest fear," he said.

Security remains a paramount issue in the last month of the Expo, which will see a number of big events and a large number of visitors, Yu said.

On Oct 1, China's top legislator, Wu Bangguo, along with a number of foreign parliamentary leaders and heads of state are expected to attend China's National Day celebration at the Expo. The closing ceremony of the event is expected to draw the country's Premier Wen Jiabao and foreign dignitaries.

As the fair approaches the end of its run, 14 million tickets remain unused, so there could be a huge number of visitors in the last days of the event.

Crowds of visitors are expected on the weekends of Oct 16 and Oct 23. Organizers want to be prepared after a record 810,000 visitors passed through the turnstiles on a single day at the 1970 Expo in Osaka.

So far, the record for daily attendance at the Shanghai Expo was 630,000 on Sept 23, when people had to wait up to six hours for a glance of the most popular halls, such as the Saudi Arabia Pavilion.

In anticipation of a surge in visitors, Yu asked Expo workers to remain vigilant until midnight on Oct 31, when the event closes.

He specifically warned about the possibility of spontaneous celebrations taking place at pavilions on the last night of the fair.

"There will be some wine left and I am afraid that some bacchanals might take place on the Expo site," he said.

For Shanghai, the Expo presents the opportunity for the city to make a global impact, similar to the way the Beijing Olympics put the capital in the spotlight in 2008.

By Thursday, more than 57 million people had visited the Expo, which is expected to attract an estimated 70 million visitors.