About 500 urban planners, civic figures, public officials and activists gathered in a historic Delray Beach schoolhouse Wednesday to launch a dauntingly ambitious -- and unprecedented -- undertaking: Mapping out a 50-year-plan for the sustainable growth of the seven Southeast Florida counties that stretch along the Atlantic from the Keys to Indian River.

The two-year effort, dubbed Seven50, is designed to produce a coordinated but voluntary action plan to address common, critical issues or needs such as population growth, suburban sprawl, transportation, economic development and environmental protection. The overarching goal is to secure the region’s economic future while improving its quality of life, organizers said.

“We get to pull the lens back far enough to have an opportunity to make the place more prosperous and a nicer place to live,’’ said nationally prominent Coral Gables-based planner Victor Dover, the project’s lead consultant.

The plan is led by a consortium established jointly by the South Florida Regional Planning Council and the Treasure Coast Regional Planning Council and is funded by a $4.25 million grant from the U.S. Department of Housing and Urban Development. Some 200 public agencies, advocates, business and academic groups are involved, including county and city planners, elected officials, major universities and chambers of commerce.

Organizers were happily schocked at the number of people who signed up for Wednesday’s summit meeting at the Old School Square in downtown Delray. Registration was shut down days early because the meeting had reached the maximum capacity of 510 people, they said. About 100 more people watched a live video stream of the meeting.

The event’s popularity reflects both a growing affinity and concern for Southeast Florida as a formerly transient or transplanted population takes root, coupled with a realization that local governments and civic groups can’t effectively tackle its problems and needs in isolation, said Michael Busha, executive director of the Treasure Coast council, which represents Indian River, St. Lucie, Martin and Palm Beach counties.

“We know there is going to be lots of growth and investment, so we might as well have a plan rather than a hodge-podge,’’ said James Murley, director of the South Florida council, which groups Monroe, Miami-Dade and Broward counties.

And hodge-podge is what the region has been to date, with a “confused scenario’’ of disconnected planning efforts, said urban affairs writer Neal Pierce, the day’s keynote speaker.

“The time seems right for fresh thinking,’’ he said. “You have a long, long way to go, but this is a good start.’’

HUD has issued similar grants to 45 U.S. regions in an effort to get more bang for its economic development dollars as federal outlays shrink amid increasing demand. HUD wants to focus its funding on places with smart strategic plans that can best take advantage of the money.

The project builds on a new body of research suggesting that the world economy will be increasingly driven by such super- or mega-regions, especially those with the most efficient transportation systems, diversified businesses, cultural assets and skilled workforces.

The Southeast Florida super-region, Dover said, is poised to become one of the most important in the world given its sea and air ports, its growing and diverse population and its international trade connections.