The 51-year-old Jackson is the court's 116th justice, and she took the place of the justice she once worked for. Justice Stephen Breyer's retirement was effective at noon.

Chief Justice John Roberts welcomed Jackson "to the court and our common calling."

Jackson, a federal judge since 2013, is joining three other women -- Justices Sonia Sotomayor, Elena Kagan and Amy Coney Barrett. It's the first time four women will serve together on the nine-member court.

Biden nominated Jackson in February, a month after Breyer, 83, announced he would retire at the end of the court's term, assuming his successor had been confirmed. Breyer's earlier-than-usual announcement and the condition he attached was a recognition of the Democrats' tenuous hold on the Senate in an era of hyper-partisanship, especially surrounding federal judgeships.

Glynda Carr, president of Higher Heights for America, an organisation that advocates for the growth of Black women's political power, said the timing of Jackson's swearing-in was bittersweet.

"Although we celebrate her today, one Black woman or a cohort of Black women can't save this democracy alone. We are a piece of it and we are doing our work, our part. She's going to forever reshape and shape that court. But she's just a piece of the work that needs to happen moving forward," Carr said.

Black lawyer Judith Browne Dianis said that even the series of conservative rulings from the court over the past week cannot take away from the significance of Thursday's ceremony.

"This is a momentous occasion and it's still a beautiful moment," said Dianis, executive director of the civil rights group Advancement Project.

But, Dianis added, "she's joining the court at a time when conservatives are holding the line and trying to actually take us back, because they see the progress that's being made in our country. It's like the Civil War that never ended. That's the court that she's joining."

Jackson helps form the most diverse court in its 232-year history and is the first former public defender to be a justice.

The court that Jackson is joining is also the most conservative that it has been since the 1930s. She is likely to be on the losing end of important cases, which could include examinations of the role of race in college admissions, congressional redistricting and voting rights that the court, with its 6-3 conservative majority, will take up next term.

The court is surrounded by fencing, and justices and their families have 24-hour protection by the US Marshals, the result of a law passed days after a man carrying a gun, knife and zip ties was arrested near Justice Brett Kavanaugh's Maryland house after threatening to kill the justice.

The bill was introduced in May shortly after the leak of a draft court opinion that would overrule Roe v. Wade and sharply curtail abortion rights in roughly half the states.

The court issued final opinions earlier Thursday after a momentous and rancorous term that included overturning Roe v. Wade's guarantee of the right to an abortion.

One of Thursday's decisions limited how the Environmental Protection Agency can use the nation's main anti-air pollution law to reduce carbon dioxide emissions from power plants, a blow to the fight against climate change.

Australian Associated Press