America's new top prosecutor, Jeff Sessions, has said the US must bring an end to illegal immigration, as he was sworn in at the White House.
The US attorney general said: "We need to end this lawlessness that threatens the public safety, pulls down the wages of working Americans."
During the ceremony, President Donald Trump signed three executive orders targeting crime and drug cartels.
The Senate confirmed him in the post on Wednesday by a vote of 52 to 47.
Who is Jeff Sessions?
Is Trump cabinet facing historic obstruction?
US soldier brings Afghan to live with him
Arrests at US protest over Mexican woman
Mr Sessions, who is widely seen as an inspiration for Mr Trump's anti-immigration policies, was appointed after a series of divisive congressional hearings over his record on civil rights.
Vice-President Mike Pence administered the oath in the Oval Office on Thursday.
"We need a lawful system of immigration," Mr Sessions said afterwards.
"One that serves the interest of the people of the United States. That's not wrong, that's not immoral, that's not indecent."
Mr Sessions also pledged to address the country's crime problem, which he called a "dangerous and permanent trend that places the health and safety of the American people at risk".
Joining his new chief prosecutor in the Oval Office, Mr Trump signed three additional executive orders on crime.
He said they were designed to "break the back of the criminal cartels that have spread across our nation and are destroying the blood of our youth".
The orders were also said to target those who commit crimes against law enforcement.
Mr Sessions, whose nomination was among Mr Trump's most controversial, resigned his Senate seat shortly after his colleagues confirmed him.
At 70, he is the same age as Mr Trump and was one of his earliest political supporters.
He will now take charge of the justice department and its 113,000 employees, including 93 US attorneys.
Allegations of racism have dogged Mr Sessions since he was rejected by the Senate in his nomination for a federal judgeship in 1986.
Senate Democrats highlighted these allegations in recent debates.
Senator Elizabeth Warren was silenced on Tuesday night after she recalled criticism of Mr Sessions by Martin Luther King's widow, who alleged he had intimidated black voters.
The Massachusetts Democrat, who was temporarily banned from the Senate, wrote: "If Jeff Sessions makes even the tiniest attempt to bring his racism, sexism & bigotry into the Justice Department, he'll hear from all of us."