The city government said in a statement that the 28-story building was being renovated when it caught fire at about 2 p.m. (0600 GMT). It did not give a cause for the fire.
Footage on state television showed clouds of smoke billowing out of the building, with flames still visible several hours after the fire started.
Shanghai state television said more than 80 fire trucks were called to battle the blaze, and streams of water could be seen flowing into the building, which appeared to be gutted.
The official Xinhua News Agency quoted witnesses as saying that scaffolding used for the renovation caught fire first and the blaze spread to the building.
The city government said 100 people were rescued, but it was not clear if that included the 90 people Xinhua said were hurt in the fire.
Photos published online showed clouds of black smoke flowing from the top of the building.
A doctor at Shanghai’s Jing’an Hospital surnamed Zhang said more than 20 seriously hurt people had been admitted for treatment.
