In what may be his strongest condemnation of the Trump administration yet, Mark Zuckerberg called the decision to end DACA "particularly cruel."

"The decision to end DACA is not just wrong," the Facebook (FB) CEO wrote Tuesday about the dissolution of a program that protect undocumented immigrants who came to the United States as children.

"It is particularly cruel to offer young people the American Dream, encourage them to come out of the shadows and trust our government, and then punish them for it," Zuckerberg wrote. "This is a sad day for our country."

Attorney General Jeff Sessions said the program, which protects nearly 800,000 young undocumented immigrants (often called Dreamers), "is being rescinded." Congress now has six months to come up with a legislative fix.

The repeal effort has worsened what was already a tense relationship between President Trump and the business community.

Related: Business leaders call on Trump to protect DACA

Ahead of the decision, more than 400 business leaders signed on to a letter from FWD.us, an immigration advocacy organization, calling for Trump and Congress to protect DACA.

Zuckerberg, a cofounder of FWD.us. urged his millions of Facebook followers to "call your members of Congress and tell them to do the right thing."

Other top tech execs were quick to criticize the decision, too.

"Dreamers are our neighbors, our friends and our co-workers," Sundar Pichai, CEO of Google (GOOGL), said in a Twitter post. "This is their home. Congress needs to act now to #DefendDACA."

Brad Smith, president and chief legal officer at Microsoft (MSFT), wrote in a blog post that the company is "deeply disappointed by the administration's decision today."

Silicon Valley has clashed with the Trump administration on a range of issues this year, including the decision to pull out of the Paris climate agreement and withdrawing protections for transgender students.

But Trump's immigration policies, including the travel bans, have been a particular lightning rod in an industry where many big companies and startups were founded by, and are still staffed with, immigrants.

Apple CEO Tim Cook tweeted Sunday that the company has 250 Dreamers on staff. Microsoft says it currently has 39 Dreamers, and a source close to IBM (IBM) says it has at least 30.

Facebook, Twitter (TWTR), Amazon (AMZN) and Google declined to provide estimates.