The incident occurred in the interior village of Keirao while the labourers were constructing the bridge on Iril river yesterday, sources said.
They said the labourers were taken to J N Hospital near here, adding the condition of them was "serious".
A PWD official said the bridge was being constructed at an estimated cost of Rs 2 crore and investigation would be conducted to find if there was any lapse on the part of the contractor engaged with the construction work.