Mayor Michael Bloomberg joined developers from Related Companies and Oxford Properties Group to break ground today on the 26-acre Hudson Yards development, a project that will greatly transform Manhattan's West Side over the next decade.

The development will be built over the Metropolitan Transportation Authority's rail yards and stretch from 30th to 33rd Streets between 10th and 12th Avenues.

It will turn the area into a mixed-use development with millions of square feet of retail, residential and office space.

Construction is now underway on the first tower, a 47-story building at 30th Street and 10th Avenue that will be the future world headquarters of handbag maker Coach.

Stephen Ross, the chairman of Related Companies, and the mayor said the project will fundamentally change the city.

"Today we start construction on Hudson Yards and prove yet again that New York City is the greatest city in the world, and will remain so," Ross said. "The tower we begin will anchor a new neighborhood that will become the new heart of New York."

"It is one of the largest private developments ever undertaken in the country, and the beautiful, state-of-the-art commercial, retail and cultural spaces that will be built here will further transform our city's iconic skyline," said Bloomberg.

The project is also expected to include a riverfront park, a school and a cultural center.

The first building is set to be finished in 2015, and the entire site should be complete in 10 to 12 years.