The NYPD plans to build a fortress around the World Trade Center site once it’s completed — and the measures have downtown residents up in arms.

The $40 million police proposal would limit traffic over a 16-acre swath of the Financial District and includes guardhouses, bollards and checkpoints, documents show.

Residents wanted Greenwich Street to be finally fully open to traffic — it was cut off by the original Twin Towers — but the NYPD rejected that idea in favor of security checkpoints on cars and trucks, documents show.

East-west traffic between West and Church streets also will be cut off by the plan, which puts security barriers on Vesey and Fulton streets and an elaborate “sally port” on Liberty Street so cops can check tour buses.

The plan was laid out in an environmental-impact statement required by city and state laws.

The deadline on comments for the proposal is next Wednesday, and the police and other agencies expect to begin construction of street barriers, security booths, sidewalk bollards and other installations later this year.

The plan also calls for bollards that would run the length of West Street from Vesey to Liberty streets and along Church Street and Trinity Place.

About seven intersections will be limited by a dual-barricade system controlled by police officers in guard stations.

“Each personnel booth would be up to approximately 11 feet tall with a building footprint measuring up to approximately 12 feet by six feet,” documents show.

Some downtown activists say the plan goes too far.

The NYPD will “close access to many of the streets that under the original plan for the redevelopment were supposed to be open,” said Julie Menin, former chair of Community Board 1.

Menin, a candidate for Manhattan borough president, was among those to oppose an early draft of the plan presented last year to the City Planning Commission.

But other experts in urban planning say the police proposal will help the neighborhood and its businesses by encouraging people to walk or bicycle.

“It’s going to be better for pedestrians and it’s going to be worse for vehicles. That’s a good thing,” said Jeff Zupan, of the Regional Plan Association.

“The bad thing is that you have these checkpoints that make it look more like an armed camp or a gated community.”