Construction has begun on the security fence for Toronto's upcoming G20 summit.

Workers were out early Monday at the foot of Simcoe Street near Lakeshore Boulevard West to place the first section of the concrete base that will make up the security perimeter.

Workers set concrete blocks in place along lower Simcoe Street in dowtown Toronto early Monday. CBC

Chain-link fencing will be installed on top of that, making the entire structure three metres tall.

Once complete, it will stretch 3.5 kilometres, encompassing an area of the downtown core that contains the Metro Toronto Convention Centre, where world leaders will meet June 26-27, as well as the CN Tower. The landmark tower will be closed during the summit.

The northern portion of the fence will run along Wellington Street West, while the southern portion will run along Lakeshore Boulevard West. The western portion zigzags southward along Windsor Street and Blue Jays Way. The eastern portion runs along Bay Street, east along Front Street West and then south on York Street.

Union Station, the Air Canada Centre and the Rogers Centre are outside the security fence.

Meaghan Grey, with public affairs for the Toronto police G20 planning team, said there will be "some impact on traffic" during construction of the barrier.

Const. Wendy Drummond, spokeswoman for the Toronto Police Service, said there will have to be some road closures wherever the fence is being assembled.

Construction will continue around the clock until it's finished. Once it's done, you'll need to show photo ID and give a valid reason for entering the zone.

When world leaders come to the city, there will be a second zone, the traffic perimeter. There will be police checkpoints from Yonge Street in the east to Spadina Avenue in the west and from King Street in the north to Queen's Quay in the south.

Authorities have outlined the security zones that will be enacted leading up to the G20 summit in downtown Toronto.