PORTLAND – A proposed $245 million development with a 17-story office building and sports arena would be a “transforming event” for Maine’s largest city while boosting the economy of the state, says the businessman who wants to build it.

Located on 6.5 acres next to Portland City Hall, the complex would be paid for with a combination of private investment and public money. Construction would begin in 2007.

It would have a 10,000-seat arena as well as a 140,000-square-foot convention center. Other buildings would include a 250-room hotel, a 300,000-square-foot office building and a parking garage with enough spaces for 1,700 cars.

The office building would rise above nearby Franklin Towers, which is considered the tallest building in Maine.

“I think it could be a real transforming event for the city that could take it to the next level,” said Joseph Boulos, who owns the real estate firm CB Richard Ellis/The Boulos Co.

Various groups and individuals have been pushing for five years for a new Portland civic center to replace the city’s aging Cumberland County Civic Center, but funding problems and neighborhood opposition have derailed those proposals.

Boulos presented his proposal Wednesday to trustees for the Cumberland County Civic Center. The site now hosts a pair of parking lots and a building owned by Blethen Maine Newspapers.

Boulos said he has support for his plan from the site’s owners and a number of prospective tenants, and has secured an option for one of the parking lot properties.

The project’s future may rest ultimately with the Legislature if state financing becomes necessary.

“This opportunity goes away if something does not happen this session,” said Kay Rand, former chief of staff for former Gov. Angus King and now a consultant for the civic center trustees.

Gov. John Baldacci plans to release a report next month that recommends a variety of funding mechanisms for public projects, such as the one proposed for Portland, that promote economic growth and tourism.