Thousands of marchers have braved freezing temperatures in Washington DC to demand an end to abortion in the US, with Pope Francis tweeting his support.
The annual March for Life marks the anniversary - in this case the 41st - of the US Supreme Court's landmark Roe versus Wade decision that effectively legalised abortion across the country.
"We're pro-life because we believe in the rights of unborn children," March for Life president Jeanne Monahan told the crowd on the snow-covered National Mall on Wednesday.
On Twitter, where he has three million followers in English, Pope Francis sent from the Vatican a message of support to the protesters, including large numbers of Roman Catholics.
"I join the March for Life in Washington with my prayers," said the pontiff, who last year denounced abortion as a symptom of today's "throw-away culture".
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"May God help us respect all life, especially the most vulnerable," he added.
US President Barack Obama, who has expressed admiration for some of the pope's messages and is due to meet him in Rome in March, is on the other side of the debate.
In a statement to mark the anniversary, the White House reaffirmed Obama's position that "every woman should be able to make her own choices about her body and her health.
"We reaffirm our steadfast commitment to protecting a woman's access to safe, affordable health care and her constitutional right to privacy, including the right to reproductive freedom," it said.
At the rally on the nearby Mall, speaker after speaker mourned the estimated 56 million fetuses which, they said, have been "brutally slayed" in US abortion clinics over the past four decades.
"Our society and our leaders must stop upholding abortion and start encouraging adoption," said Republican congresswoman Vicky Hartzler.
"Abortion hurts everyone," she declared.
The issue returned to the Supreme Court last week when its nine justices heard arguments over a Massachusetts law creating a buffer zone around abortion clinics targeted by pro-life activists.
Pro-abortion advocates cancelled plans for a counter-protest in Washington DC on Wednesday, citing the cold snap that followed a snowstorm on Tuesday up and down the northeastern US coast.
