Shanghai is to present a "quality" World Expo despite challenges posed by its unprecedented size and an expected record high number of visitors, said chief of the International Exhibitions Bureau (BIE) on Wednesday.

A concern for the BIE and the Expo's organizers is "how to balance the largest participation ever with a quality exposition," said Vicente Gonzalez Loscertales, secretary-general of the BIE, at a press conference.

Eight years ago when the organizing work began, "we knew we had to face challenges at an unexpected scale," and problems were expected to emerge during the Expo which would see "the largest park, the largest number of visitors and the largest number of participants", he said.

However, in two days when the Expo opens doors to the world, "visitors will see that this challenge of quantity and quality had been very successfully met", he said.

Loscertales attributed the success to full support from the Chinese central government and the Shanghai organizers.

Some 189 countries and 57 international organizations would take part in the Shanghai Expo, which features some 189 National Pavilion Days and 39 International Organization Days as well as 20,000 cultural performances.

The six-month event is expected to attract over 70 million visitors from both home and abroad.