Millions of women in the US will lose the constitutional right to abortion, after the Supreme Court overturned its 50-year-old Roe v Wade decision.
The judgement paves the way for individual states to ban the procedure.
Half are expected to introduce new restrictions or bans. Thirteen have already passed so-called trigger laws to automatically outlaw abortion.
President Joe Biden described it as “a tragic error” and urged states to enact laws to allow the procedure.
After the Supreme Court ruling, abortion access is expected to be cut off for about 36 million women of reproductive age, according to research from Planned Parenthood, a healthcare organisation that provides abortions.
Demonstrators from both sides had gathered outside the court, with police keeping them apart.
One anti-abortion activist told the BBC she was “elated” as her side cheered the decision. “It’s not enough just to make this the law of the land. To be pro-life is to make [abortion] unthinkable,” she said.
Across the divide, pro-choice supporters decried the decision as “illegitimate” and even a form of “fascism”.
The landmark 1973 Roe v Wade case saw the Supreme Court rule by a vote of seven to two that a woman’s right to terminate her pregnancy was protected by the US constitution.
The ruling gave American women an absolute right to an abortion in the first three months (trimester) of pregnancy, but allowed for restrictions in the second trimester and for prohibitions in the third.
But in the decades since, anti-abortion rulings have gradually pared back access in more than a dozen states.
In its current session, the Supreme Court had been considering a case, Dobbs v Jackson Women’s Health Organization, that challenged Mississippi’s ban on abortion after 15 weeks.
By ruling in favour of the state, the conservative-majority court effectively ended the constitutional right to an abortion.
Five justices were firmly in favour: Samuel Alito, Clarence Thomas, Neil Gorsuch, Brett Kavanaugh and Amy Coney Barrett.
Chief Justice John Roberts wrote a separate opinion saying that, whilst he supported the Mississippi ban, he would not have gone further.
The three justices who disagreed with the majority – Stephen Breyer, Sonia Sotomayor and Elena Kagan – wrote that they had done so “with sorrow – for this court, but more, for the many millions of American women who have today lost a fundamental constitutional protection”.
Friday’s ruling amounts to a wholesale reversal of the Supreme Court’s own legal precedent – an extremely rare move – and is likely to set up political battles that divide the nation.
In states where opinions on abortion are closely split – such as Pennsylvania, Michigan and Wisconsin – the legality of the procedure could be determined on an election-by-election basis. In others, the ruling may set off a new round of legal battles, including over whether individuals can go out of state for abortions or order abortion drugs through mail services.
Denouncing the Supreme Court ruling, President Biden told women in states where it was banned to travel to those where it was not.
Democratic governors of several states including California, New Mexico and Michigan have already announced plans to enshrine abortion rights within their constitutions.
Governor of Mississippi Tate Reeves quickly welcomed the ruling, saying his state had “led the nation to overcome one of the greatest injustices in the history of our country”.
“This decision will directly result in more hearts beating, more strollers pushed, more report cards given, more little league games played, and more lives well lived. It is a joyous day!” he wrote.
Former Vice-President Mike Pence, a long-standing critic of Roe v Wade, urged supporters not to stop until “the sanctity of life” was protected by law in every state.
On the other side of the divide, Democratic Speaker of the House Nancy Pelosi said that “the Republican-controlled Supreme Court” had achieved that party’s “dark and extreme goal”.
“American women today have less freedom than their mothers,” she wrote. “This cruel ruling is outrageous and heart-wrenching.”
The reversal of a long-standing precedent has also raised fears for other rights decided upon by the Supreme Court in the past.
Justice Clarence Thomas, in his opinion, wrote: “In future cases, we should reconsider all of this Court’s substantive due process precedents, including Griswold, Lawrence, and Obergefell” – referencing three landmark decisions of the past on the right to contraception, the repeal of anti-sodomy laws, and the legalisation of same-sex marriage respectively.