The skirmishes erupted when protesters occupying the square for nearly two weeks tried to expel the peddlers, but the fighting quickly degenerated into clashes with both sides lobbing rocks and molotov cocktails, witnesses said.
The violence took place hours after Egyptians wrapped up two days of peaceful polling in the first phase of multi-stage elections for a new parliament.
Most of the injuries were treated on the spot but 27 people were admitted to hospital, the health ministry said.
Protesters have been occupying the square to demand the ouster of the military junta which took power when Hosni Mubarak was overthrown in a popular uprising in February.
– SAPA
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