The new checkpoints at bridges and tunnels will stop cars and seek to enforce quarantine orders imposed by New York State, which require people coming from 34 states and Puerto Rico to self isolate for up to 14 days to avoid spreading Covid-19.

The city Sheriff's Office will run the checkpoints and stop a random sampling of cars entering the city, reminding drivers of the quarantine order and requiring them to fill out a registration form if they're coming from a high risk state.

People caught violating the quarantine — which applies to both residents of the high-risk states and New Yorkers returning from visits — can be hit with fines up to $10,000.

"The checkpoints, I think, are going to send a very powerful message that this quarantine law is serious," Mayor Bill de Blasio said.

The city, where more than 23,000 people have died from Covid-19, has kept its infection rates low in recent months but is guarding against a resurgence as cases spike across much of the country and it looks to reopen schools in about a month.

One fifth of new Covid-19 cases are now coming from out-of-state travelers, said Ted Long, who heads the city's contact tracing program.

"If you come here, you must quarantine. It is not optional," de Blasio said. "We do not want to fine you. We do not want to penalize you. In fact, we want to help you quarantine. But if you don't respect our laws, we will penalize you."

The city's Test & Trace Corps will also deploy to Penn Station on Thursday to stop travelers, remind them of the quarantine rules and make sure they fill out the traveler health form.

The state has been focusing its enforcement efforts on airports.