Police fire tear gas at Istanbul's Taksim Square protestors

Riot police fired tear gas at hundreds of demonstrators on Friday, injuring at least two dozen people, in a bid to break up a four-day protest against a major construction project in Istanbul's iconic Taksim Square.

Several of the wounded were left lying on the ground unconscious after they were hit with large quantities of tear gas and pepper spray, while two people were hospitalised with injuries to the head, an AFP photographer witnessed.

Two protestors suffered broken arms and several others had minor bone fractures when a scaffolding collapsed as they tried to escape the police intervention on the square.

Frequent skirmishes were breaking out between the police and dozens of young protestors, who shouted: "You are killing us!," and hurled rocks at the security forces.

The gas infiltrated the subway and through the windows of passing cars, as ambulances arrived to carry away those affected.

"They are spraying anybody like it is pesticide," tweeted one protestor using the handle @blogcuanne, "Kids, babies, the old, tourists, nobody matters."

Thousands of others have voiced their support for the protestors on social media, while Amnesty International issued a statement on Thursday condemning use of "excessive force against peaceful protestors."

Construction had began in November to pedestrianise the zone surrounding the famous square, a traditional gathering point for rallies and protests as well as a popular tourist destination.

The controversial project is aimed at easing the chronic congestion in the roads around the square.

Demonstrators have been trying to prevent workers from razing Taksim Excursion Park, which lies across from the square's centrepiece, the Ataturk monument. In place of the park, a shopping mall is to be built.

Critics say the project would turn the square into yet another soulless concrete commercial zone aimed at making money while driving away residents who use it as a meeting point.

Taksim Square has for decades been the rallying point for millions of Istanbul residents, as well as the political stage for demonstrators who pour in on a daily basis to make their views on different causes heard.