The concrete slab collapsed at Sujanpura near the Alambagh bus station, said the police.
The injured, said to be labourers, have been admitted to a nearby hospital, where condition of one is stated to be critical. Others are being treated for non-life threatening injuries.
District Magistrate of Lucknow Raj Shekhar told IANS that there were “no casualties reported as of now,” meaning the accident has not resulted in any loss of life.
Senior district and police officials are at the accident site, overseeing rescue operations.
Shekhar said that 15 personnel of the National Disaster Relief Force (NDRF) have reached the accident site to help in rescue operations.