The Games start on July 26 with an opening ceremony that will see thousands of athletes and performers paraded along a 6-km (4-mile) stretch of the river.
Organisers promise an unprecedented extravaganza, but it is one fraught with security risks.
Metal fences were erected on Thursday. Anybody living or working inside the perimeter requires a pass with a QR code to get through checkpoints.
Some Parisians revelled in the calm, others complained about the new restrictions.
Luxury sector worker Estelle Boubault said the unusual quiet inside the restricted zone reminded her of lockdowns during COVID. "But it's a bit more joyous than COVID, it's the Olympics!" she said.
Departing from the Austerlitz bridge, the parade will sail by Notre-Dame de Paris cathedral and arrive near the Eiffel Tower, after passing under bridges and gateways, including the Pont des Arts and Pont Neuf, and near many of the French capital's landmarks.
Organisers have said there will be music, dance and performances on every riverbank and bridge.
But while President Emmanuel Macron's desire to put the city centre stage will produce great images, it will also require a security operation never before seen in peacetime France.
Some 45,000 police will be deployed to ensure the ceremony's security, including special intervention forces. Snipers will be deployed on rooftops, an anti-drone system will be in place, and the river will be checked for submerged bombs.
Paris authorities have been telling residents for months they will need a pass. Newly-arrived tourists were caught off-guard. Some took photos of famous landmarks through metal grills.
Company executive Sebastian Bouleau said there was some confusion on the first day, and that the police did not all seem to be giving the same instructions.
"I think we just need an adjustment period," he said.
