Construction began Wednesday on an upgraded museum and visitor center at St. Louis' iconic Gateway Arch as part of an ambitious $380 million bid to renovate the grounds of the towering landmark on the cusp of turning a half century old.

Dignitaries including Mayor Francis Slay and the National Park Service's deputy chief broke ground on a planned 46,000-square-foot expansion of the visitor center and a redesigned, modernized Museum of Westward Expansion, both located underneath the 630-foot-tall monument.

The makeovers, overseen by the park service, are expected to be completed in about two years.

"The Arch is the undeniable symbol of our region, recognized around the world. With this project we are better connecting it to the city it represents," said Slay, who joined nonprofit CityArchRiver 2015 Foundation — the renovation project's coordinator — during Wednesday's ceremony.

More than 900 donors — from businesses to individuals, families and regional foundations — have contributed roughly $212 million in private funding for the project, about $9 million short of the fundraising goal, CityArchRiver said. Other funding has or will come from a potpourri of sources including state tax breaks, grants, some sales tax revenues and the park service.

Drawing about 2.5 million annually, the Arch along the Mississippi River's western bank was completed 50 years ago and opened to visitors in 1967 as a monument to President Thomas Jefferson and pioneers for whom St. Louis served as the gateway to the West.

The Arch ground's makeover also includes ongoing efforts to create a park atop a concrete-and-steel lid being built over a interstate highway between the arch and the city's downtown, thereby expanding the green space.