Mayor Bill de Blasio’s plan to pedestrianize streets around Rockefeller Center this holiday season will only cause “city-wide gridlock river to river” and lead to a slowdown in emergency response times, the city’s firefighters union warned on Sunday.

The FDNY-Uniformed Firefighters Association shot back against the mayor’s announcement on Friday that the tourist mecca will be closed to cars from Nov. 29 through the New Year in an effort to safeguard the crush of pedestrians flocking to the area.

“The move to increase ‘pedestrian space’ surrounding Rockefeller Center is misguided and makes this city less safe — plain and simple,” said union president Gerard Fitzgerald in a statement. “As it is, traffic is interfering with our firefighters’ abilities to reach the scene of a fire, but this new plan will have wide-felt repercussions in the form of traffic from river to river.”

Fitzgerald said the closures — scheduled for 49th and 50th streets between Fifth and Sixth avenues from 2 p.m. to midnight Monday through Thursday, 1 p.m. to midnight on Friday and 10 a.m. to midnight on the weekends — will interfere with emergency vehicle traffic and will prohibit smoke eaters and other first responders from reaching emergency scenes.

He added that the “city failed to adequately notify the companies most affected by the changes.”

“We all want pedestrian safety, but a key component of safety for all is the ability for New York’s first responders to reach the scene of an emergency, and this is not the best way to ensure the safety of New York’s over 8 million residents and millions of holiday visitors,” Fitzgerald said.

City Hall spokesman Will Baskin-Gerwitz said keeping New Yorkers safe during the holiday season was of “the utmost importance.”

“The NYPD and Department of Transportation employees on the ground at Rockefeller Center will have the flexibility to re-open lanes of traffic as necessary in the event of an emergency,” he said.

FDNY spokesman Frank Dwyer added, “As has happened every year during the holiday season, the department will work closely with NYPD and DOT to ensure both the safety of New Yorkers and visitors, and that our members can quickly respond to emergencies in and around Rockefeller Center.”