One of the many executive orders President Donald Trump signed on Monday included one to end birthright citizenship. Here's what to know about it and why lawsuits have already been filed.

The lawsuit to block the president’s executive order is the first salvo in what is likely to be a long-running legal fight over immigration policy.

The executive order restricts automatic citizenship for babies born to parents who aren't citizens or lawful, permanent residents.

Residents in Hackensack, New Jersey gathered Tuesday to protest President Donald Trump's executive order ending birthright citizenship.

Immigration rights groups, including the American Civil Liberties Union, sued the Trump administration soon after the president signed an executive order on immigration that would end birthright citizenship for children born in the U.

"We think we have really good grounds," the president said of the order, which has already been challenged in federal court.

"Birthright citizenship in our country is a guarantee of equality, born out of a collective fight against oppression," Attorney General Andrea J. Campbell said in a statement.

Attorneys general in 18 states sued Tuesday to block the order, saying birthright citizenship cannot be undone with the stroke of the president’s pen.

President Trump promised Monday to take aggressive action to curb the number of undocumented migrants in the US, and one of the bigger initiatives revolves around what's known as birthright citizenship.