New York Mayor Bill de Blasio (D) says his city will sue President-elect Donald Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE’s incoming administration if it implements a national registry for Muslims.

“We will sue to block it,” he said at New York City’s Cooper Union Monday, according to the New York Daily News.

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“We will use all the tools at our disposal to stand up for our people,” added de Blasio, who backed Democratic presidential nominee Hillary Clinton Hillary Diane Rodham ClintonButtigieg stands in as Pence for Harris's debate practice Senate GOP sees early Supreme Court vote as political booster shot Poll: 51 percent of voters want to abolish the electoral college MORE. "[We must] always be proud of our values."

De Blasio called on listeners to organize “issue by issue” with other Americans and halt Trump’s policies they disagree with.

“We will make sure women receive the healthcare they need,” he said as an example, referencing possible Planned Parenthood funding cuts.

De Blasio added that New York City would defy the Trump administration if it demanded the city escalate stop-and-frisk tactics for its police.

The mayor also reiterated he would not allow the New York Police Department to be used for deporting undocumented immigrants.

De Blasio noted his government would provide expanded legal aid for undocumented immigrants in New York City facing such deportations.

A top ally of Trump’s said last week that the Republican is mulling a national registry for immigrants from Muslim countries or nations with significant terrorism struggles.

Kansas Secretary of State Kris Kobach (R), an immigration hard-liner who is advising Trump, said the billionaire’s transition policy advisers are weighing such a database.

Trump’s transition team has since repeatedly denied that the registry would be based on religion.

Reince Priebus, Trump’s future chief of staff, on Sunday pledged the administration would not register people based on their faith.

“Look, I’m not going to rule anything out,” Priebus, the chairman of the Republican National Committee, said on NBC News. "But ... we’re not going to have a registry based on religion."