Thousands of angry demonstrators gathered outside Sen. Chuck Schumer’s luxury Brooklyn apartment building — holding up signs and chanting “What the f–k, Chuck?!” — to protest his lukewarm stance on President Trump’s cabinet picks.

“Senator Schumer needs to know we’re watching him,” fumed Brad Wolchansky, a 40-year-old soccer coach from Flatbush who was carrying a cardboard cutout of a giant eye on Tuesday night.

“He works for us. We need him to be bold. We need him to stand up to Trump and oppose his picks,” Wolchansky said.

He and roughly 3,000 others gathered at Grand Army Plaza around 6 p.m. before eventually making their way over to Schumer’s building at 9 Prospect Park West, between Carroll and President streets.

As they marched, the anti-Trump demonstrators waved signs saying “Resist Trump” and “Show Some Spine Schumer” — while also chanting things like “Stay strong, Chuck” and “Shut it down, shut it down, New York is an immigrant town.”

The group is ultimately up in arms over what they refer to as Schumer’s “centrist” beliefs and willingness to play ball with the president.

While Schumer did say Monday that he would oppose at least eight of Trump’s cabinet nominees, he has already voted “yes” on three of his picks: Gen. James Mattis for the Department of Defense, Gen. John Kelly for the Department of Homeland Security and Mike Pompeo for the CIA.

“He’s talking the talk on social media, but is he walking the walk?” said Cambra Moniz-Edwards, 35, of Brooklyn. “What the f–k, Chuck?!”

Phoebe Damrosch, 38, of Brooklyn, cited Schumer’s headline-grabbing tears during a recent news conference on Trump’s immigration ban as she voiced her concern Tuesday night.

“His tears know what’s right,” the mother of three said, while carrying a sign with a pack of Kleenex tissues attached that said: “It takes a real man to cry and lead.”

Dara Barr, a 40-year-old acupuncturist from the East Village, told The Post she felt it was time for Schumer to crack the whip on the Democratic caucus — so she brought a black leather whip from home to shake like a pom-pom.

“I’m trying to whip Chuck Schumer into shape so he can whip those ‘no’ votes,” Barr said.