Workers were unable to start work at Bonn Square, in Oxford, this morning after arriving to discover a protester at the site.
Contractors had been expected to begin the £1.5m redevelopment of the city centre square this morning.
Police were called at around 7.30am by Oxford City Council after spotting the protester up in the branches of one of the trees.
Officers attempted to encourage the protester to come down, but he refused and suggested he would stay put.
A police spokesman said: "If work commences today they will start where it is safe to do so. If he is still refusing to come down and needs to be taken away, police will take appropriate action."
It is thought the protester is opposed to the work which will involve knocking down four trees in the square.
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