The ceremony took place on the main stage of the 25-hectare Expo site in the costal city, located about 450 km south of the Korean capital Seoul.
A total of 104 countries and 10 international organizations, including the United Nations, have confirmed participation to the 93-day Expo focusing on the theme "Living Oceans and Coasts."
"Expo 2012 Yeosu is, through its powerful theme, an urgent call of attention to the oceans and coasts, as leading entities that support life on the Planet," said Vicente Gonzalez Loscertales, Secretary General of the BIE during the speech at the ceremony.
"This is an opportunity to bring to the citizens of the world a better understanding of the oceans and coasts for the benefit of humankind. " he added.
President Lee said in his speech that Yeosu Expo is the first Expo focusing on oceans and environment. The pollution of ocean is critical; the maritime resources are reducing significantly, that's why the Expo will issue the Yeosu Declaration to remind the world this problem.
"As Vancouver in Canada, which grew into a world-class city after the 1986 Expo, Yeosu and the southern coast will certain to be reborn as a top-class resort and tourist destination," he added.
All the flags of official participants, together with the Korean national flag, the BIE flag and the Expo flag were brought onto the stage.
Korean pop stars and groups including Big Bang, 2NE1, IU as well as word's famous prima donna Sumi Cho also performed for the 2,400 guests.
The Expo has conducted three rehearsals on April 28, May 2 and May 5 receiving about 200,000 visitors during these three days.
The Expo is expecting several million visitors, including up to 500,000 foreign guests mainly from China and Japan, until August 12th.