Concrete barriers have been installed in part of Martin Place in Sydney's CBD, in the first part of a plan to install barricades along the length of the pedestrian mall.

Workers began placing the long, narrow concrete blocks between Phillip Street and Elizabeth Street on Friday, a length of Martin Place which is next to the Lindt Cafe.

Some steel bollards already exist in the area, however they are limited to part of the pedestrian walkway where it dips to meet the road.

In a statement, a spokeswoman for the City of Sydney said the temporary barriers were "to block vehicle access in response to security concerns", however there had not been a specific threat made.