The project, in the north of the city, is one of many infrastructure developments in which Erdogan takes great pride and serves in political campaigns.
Television channels were broadcasting the ceremonies live, with the politicians' speeches carried in full.
The newly built Yavuz Sultan Selim bridge.
Thousands of supporters of the ruling Justice and Development Party (AKP) and Erdogan were brought to the stage area, wearing matching white hats with the bridge's logo waving the red national flags.
The bridge is being touted by the government as the widest such suspension bridge in the world. It has eight lanes and train tracks in the middle.
The bridge is meant to help alleviate the city's traffic problems, develop the northern areas and provide a connection to the new airport, which is currently under construction.
Construction began more than three years ago and was completed at a cost of about $3bn.
The bridge project was announced in 2012 on May 29, the anniversary of the date the Ottomans conquered Istanbul in 1453. It is named after Sultan Selim, who expanded the empire greatly and died in 1520.
