Zaragoza, Spain - The northern Spanish city of Zaragoza was Friday preparing to inaugurate the World Expo 2008, the first world exhibition held in Europe in eight years, on the topic of "Water and Sustainable Development."

Some 5,000 guests including King Juan Carlos, Prime Minister Jose Luis Rodriguez Zapatero and European Commission President Jose Manuel Barroso were expected at the evening inauguration, after which the Expo will open its doors to the public for three months.

More than 100 countries are participating in the event which will focus on the vital importance of water, climate change, the unequal access to clean water and solutions to such problems.

Nearly 2,000 experts will debate water-related issues, while the expected 3.5 million visitors will be provided with entertainment in the form of about 5,000 music, dance, theatre and other spectacles.

The Expo site on the shore of the River Ebro measures 25 hectares, including a conference centre and highlights such as the 78-metres- high Water Tower and a ceramic forest at the Spanish pavilion.

Attractions include what is being billed as Europe's biggest sweet water aquarium, with 5,000 fish and other marine animals from the world's greatest water systems.

Zaragoza, a city of 650,000 residents, has extensively overhauled its infrastructure, ranging from a new airport terminal to a 280- metre pedestrian bridge by Iraqi architect Zaha Hadid, which is part of the Expo installations.

The Expo and related infrastructural works have cost public and private investors more than 3 billion euros (4.5 billion dollars).

The Expo has elicited some criticism from environmental groups, who slam Spain's own water policies as wasteful and ineffective, and criticize measures such as the construction of a barrage for leisure boats in the Ebro in view of the Expo.

The environmental group Greenpeace refused to join more than 200 other non-governmental organizations participating in the Expo. (dpa)