Works began last month to build five large concrete blocks outside the historic Hofburg palace which houses the offices of the chancellor and the president.
The barriers -- each eight metres long, 80 centimetres high and one metre wide -- were intended to withstand potential ramming attacks by vehicles, used as weapons in terror attacks across Europe over the past year.
But the €1.5 millio ($1.8 million) project sparked outrage among political opponents of Chancellor Christian Kern, ahead of a national election on October 15th.
Critics including the far-right Freedom Party (FPÖ) accused Kern, a Social Democrat, of double standards.
Vienna had refused to put up anti-terror structures in key tourist areas after jihadists killed 16 people using vehicles and knives in Spain last month.
Officials had dismissed the barriers as "placebo measures".
"Why them and not us?" read an angry headline in the Krone daily, Austria's bestselling newspaper, earlier this week.
"The public was repeatedly reassured that safety measures were currently not necessary for busy pedestrian zones and would be too much effort. What are they afraid of?" the paper said.
While Austria has been spared the major attacks that have hit France, Belgium, Britain and Germany, the authorities have bolstered anti-terror
operations since 2014.
Security and migration are key themes in next month's legislative ballot, which is being closely fought between the poll-topping conservatives, the Social Democrats and the FPÖ.