US President Donald Trump has told anti-abortion campaigners there that abortion law is "wrong" and "has to change".
He addressed the annual March for Life on Friday via video link from the White House Rose Garden.
His address was broadcast to the campaigners gathered on Washington's National Mall.
Vice-President Mike Pence spoke ahead of Mr Trump, calling him "the most pro-life president in American history".
Mr Trump said: "Today I am honoured and really proud to be the first president to stand with you here at the White House to address the 45th March for Life; that's very, very special."
Speaking to people attending the rally, he said: "You all come for one beautiful cause: to build a society where life is celebrated, protected and cherished.
"You love every child, born and unborn, because you believe that every life is sacred, that every child is a precious gift from God," he added.
The president reiterated claims he has made previously that abortions can be carried out as late nine months in some US states.
"It is wrong and has to change," he said.
"Americans are more and more pro-life, you see that all the time," he added.
"Under my administration we will always defend the very first right in the Declaration of Independence, and that is the right to life."
Mr Trump said that during his first week in office, he reinstated the Mexico City policy, first put in place by President Ronald Reagan.
This is a government policy that blocks federal funding for non-governmental organisations that provide or promote abortion as part of family planning.
Committed to "protecting the life of the unborn"
Mr Trump added: "I strongly supported the House of Representatives' pain-capable bill, which would end painful late-term abortions nationwide, and I call upon the Senate to pass this important law, and send it to my desk for signing."
Mr Trump is only the third sitting US President to address anti-abortion campaigners.
President Ronald Reagan, a Republican, made supportive remarks to the march in 1987 via telephone, while Republican president George W Bush did the same in 2003 and 2004.
Ahead of the rally, White House press secretary Sarah Sanders said Mr Trump was committed to "protecting the life of the unborn", adding that he was excited to be a part of the event.
Mr Trump was previously a supporter of women's access to abortion, saying in a 1999 interview when he was still a celebrity real-estate businessman that while he "hated the concept of abortion" he was "very pro-choice."
During the 2016 presidential campaign, as a candidate, Mr Trump said his position had "evolved," describing himself as "pro-life with exceptions," such as in cases of rape or incest.