Tens of thousands of New York City employees — including police, fire and sanitation workers — could face unpaid leave Monday as a contentious vaccine mandate goes into effect.

The consequences could be dramatic on the essential services the city depends on: Some 26,600 employees subject to the mandate hadn’t received a shot as of Friday, and already firefighters are calling in sick in higher numbers and trash has been reported piling up on sidewalks.

Tensions are high, with Mayor Bill de Blasio saying he won’t back down and city workers protesting in front of his home at Gracie Mansion. Fire Commissioner Daniel Nigiro on Saturday said that some firefighters were already engaged in a work slowdown.

“Irresponsible bogus sick leave by some of our members is creating a danger for New Yorkers and their fellow firefighters,” Nigirio said in a statement, without providing details. “They need to return to work or risk the consequences of their actions.”

Since the mandate was announced, the city had been offering $500 for any employee who got a first shot. That incentive expired Friday afternoon, and as the weekend neared, many more workers got shots.

The vaccination rate in the sanitation department jumped to 77% on Friday from 67% on Thursday. The rate for the police department increased to 84%, while the fire department’s rate rose to 77%.

While more workers likely got vaccinated over the weekend, the city has been preparing for labor shortages.

Police Commissioner Dermot Shea said on Fox 5 New York on Friday that “thousands” of officers had submitted applications for “reasonable accommodations” instead of vaccination. Officers who did so before Wednesday would be allowed to work, but would undergo testing while their claims were judged, he said.

“We’re going to be OK,” Shea said. “We have contingency plans.”

New York City has been among the more aggressive local governments in its mandates, and de Blasio has urged other employers to follow in step. The city had earlier required health workers and schools staff to get vaccinated, and has now expanded the mandate to all municipal employees.

Unlike for filling teacher shortages where there’s a need to find substitutes, the city has other options to address gaps in public safety agencies, like using overtime and redeployments, de Blasio said.

Shea said that police training had been suspended to free up more officers. Ahead of the mandate, the sanitation department has cancelled days off, will require Sunday shifts as needed and move to a 12-hour schedule for workers.

When asked about the consequences of staffing gaps, de Blasio has repeatedly said the agencies are prepared and have experience managing shortages after living through the height of the pandemic.

Some union leaders have painted the requirement as a betrayal of workers and have called its implementation haphazard and chaotic.

But city workers’ legal challenges have so far been mostly unsuccessful. The police union lost a bid to block the requirement and is appealing. Courts earlier rejected suits by teachers seeking to halt the mandate that applied to them.

Eric Adams, the Democratic mayoral candidate who’s favored to win in the Nov. 2 election, has said he thinks the mayor made the right move with the mandate, but would have first sat down with unions and addressed questions.

De Blasio said the city did talk with unions, but in the end “it was clear that we needed to move forward, whether that was going to be warmly embraced or not.”

“We needed to move forward for the health and wellbeing of our employees and the people they serve,” he said.

New York is not the only city facing a backlash for its approach with vaccines.

Chicago has required its city employees to report their vaccine status or be subject to weekly testing through the end of the year, when all employees without medical or religious exemptions are required to get vaccinated.

The deadline for reporting was Oct. 15, but some police and fire department employees have pushed back on the policy, with 72% of police department employees and 87% within the fire department in compliance with the reporting requirement as of Thursday, according to city data. Some aldermen sought to repeal the policy, but Chicago’s city council voted against the measure on Friday.