The sweeping ruling by the court, with a 6-3 conservative majority, was set to alter American life, with nearly half the states considered certain or likely to ban abortion.

Conservative Justice Clarence Thomas suggested the court's reasoning could also lead it to reconsider past rulings protecting the right to contraception, legalising gay marriage nationwide, and invalidating state laws banning gay sex.

As Saturday progressed, the number of demonstrators outside the Supreme Court increased substantially. The fenced-off area in front of the high court was filled largely with those demanding abortion rights.

Crowds carried posters with slogans such as "Abort SCOTUS". One protester carried a placard that said "limit guns, not women" in reference to another Supreme Court decision this week expanding gun rights.

Earlier in the afternoon, a supporter of Friday's ruling said: "The thing that 'my body, my choice' advocates don't get is that the aborted baby never had a choice."

The man, who identified himself as Adam John, added, "The life in the womb matters, doesn't it."

President Joe Biden, who had harsh words on Friday for the Supreme Court's decision, said the White House would monitor how states enforced bans, with administration officials having already signalled they plan to fight attempts to prohibit a pill used for medication abortion.

"The decision is implemented by states," Biden said. "My administration is going to focus on how they administer and whether or not they violate other laws."

The White House said it also would challenge any efforts by states to restrict women's ability to travel out of their home state to seek an abortion.

At a rally in Illinois on Saturday, former president Donald Trump praised the Supreme Court ruling, calling it "a victory for the constitution, a victory for the rule of law and above all a victory for life". He reminded his audience that in 2016 he made a campaign promise to nominate judges "who would stand up for the original meaning of the constitution".

For Christian conservatives who had long fought to overturn Roe, Friday's ruling was a cherished win and in part the result of a long campaign for installing anti-abortion justices to the top court. The ruling had the support of all three justices appointed by Trump.

It is at odds with broad public opinion. A Reuters/Ipsos poll last month found that about 71 per cent of Americans - including majorities of Democrats and Republicans - said decisions about terminating a pregnancy should be left to a woman and her doctor, rather than regulated by the government.

That support is not absolute: 26 per cent of respondents polled said abortion should be legal in all cases while 10 per cent said it should be illegal in all cases, with the majority supporting some limits.

The ruling will likely influence voter behaviour in the November 8 midterm elections, when Biden's Democrats face the risk of losing their razor-thin majorities in the House of Representatives and possibly the Senate.

Some party leaders hope the decision will win over suburban swing voters, though activists expressed demoralisation at suffering such a defeat while their party held the White House and Congress.

The abortion decision came one day after the court issued another landmark ruling finding that Americans have a constitutional right to carry a concealed gun for protection, leading them to invalidate a New York state law that set strict limits on concealed carry permits.

The two rulings showed an aggressively conservative court ready to remake American life at a time when Congress is often deadlocked and struggles to pass major policy changes.

Australian Associated Press