OSAKA (Kyodo) -- The World Exposition in Osaka opened Sunday, kicking off a six-month event centered on designing a sustainable future society, in what organizers hope will boost the domestic economy despite concerns over late-running preparations.

The expo will continue until Oct. 13 on Yumeshima, an artificial island in Osaka Bay, with 158 countries and regions participating. Some 28.2 million people are projected to visit.

Its major exhibits are encircled by the Grand Ring, which has a circumference of 2 kilometers and is recognized by Guinness World Records as the world's largest wooden architectural structure.

At the Japan pavilion, main exhibits include a "Mars rock" discovered by a Japanese research team in Antarctica in 2000, while the Osaka prefectural and city government's health-themed pavilion features sheets of heart muscle made from iPS cells.

Another major draw will be the 42 "Type A" pavilions by exhibiting countries including the United States and China.

The first public day comes after test-runs at the site earlier in the month. An opening ceremony featuring dancing, singing and light show was held on Saturday with 1,300 people including Emperor Naruhito, Empress Masako and Prime Minister Shigeru Ishiba attending.

The government estimates the event will have an economic impact worth 2.92 trillion yen ($20 billion) nationwide, based on the visitor target of 28.2 million people. On peak days, 220,000 people are projected to visit.

Anticipation for the event has been dampened, however, by headlines over unfinished pavilions and ballooning costs. The bill for the event has nearly doubled from an initial estimate to 235 billion yen.

Delays in pavilion construction have left organizers rushing to meet the opening date deadline. Five countries, including Nepal, which has been preparing a Type A pavilion, were unable to have their exhibits ready in time, according to the organizers.

Advance ticket sales have been slower than expected. Data current to Wednesday showed around 9 million advance tickets had been sold for the event since November 2023, far off the organizers' 14-million target.

Japan has hosted the World Exposition twice before, most recently in Aichi Prefecture in 2005. Osaka previously held it in 1970, an event that drew more than 64 million primarily domestic visitors and came to be seen as symbolizing Japan's rise as an economic power.