Construction work on the Mersey Gateway project was declared under way by the leader of Halton Borough Council earlier.
The project begins with the building of access roads across the saltmarsh on both sides of the river.
Work on the six-lane toll bridge, linking Runcorn and Widnes, is due to be finished by 2017. It aims to ease congestion on Silver Jubilee Bridge.
The existing bridge, which has been in place since 1961, will be tolled as part of the project.
Merseylink Consortium said the project will create the equivalent of 470 full-time jobs.
The budget for the total cost of construction and 30-year operating and maintenance of the project is close to Â£2bn, Merseylink Consortium and Halton Borough Council said.
The building of a second bridge over the River Mersey has started.