"Life is winning again in America," Pence told the demonstrators on the National Mall, near where Trump was sworn in a week ago before hundreds of thousands.
The March for Life also is taking place in the same area where even more massive crowds flooded Washington a day after Trump's inauguration in favor of women's rights, including abortion rights.
Pence, a longtime hero of the anti-abortion movement, is the most senior government official ever to speak in person at the rally, organizers said. As governor of Indiana, he signed what were seen as some of the nation's strictest abortion laws.
Pence, accompanied at the rally by his wife and daughter, celebrated "the election of pro-life majorities in the Congress of the United States of America," Trump's upcoming nomination of an anti-abortion Supreme Court justice, and the president's reinstatement on Monday of a policy that cuts off U.S. funding to healthcare providers that promote or provide abortions overseas.
"It's the best day I've ever seen for the March of Life," he said.
Pence said he was asked by Trump to attend the rally and thank the crowd for its support.
Trump senior aide Kellyanne Conway was one of many women to address the demonstrators before they started their march from the Mall to the U.S. Supreme Court, about 1.5 miles (2 km) away.
"We hear you. We see you. We respect you," Conway said. "And we look forward to working with you."
There were no official crowd estimates but lines of school groups and people from across the country waited to get through security checkpoints near the Washington Monument and within sight of the White House.
Protesters hoisted signs saying "Choose life," "I am the pro-life generation," and "Equal Rights For Unborn People."
A Christian rock band warmed up the crowd for Pence, leading people in rhythmic hand-clapping.
"We're here to stand up for the unborn because no one else can, and having Donald Trump in the White House makes everyone more enthusiastic," said Jim Kolar, 59, of West Palm Beach, Florida. "It's not just Trump, it's Mike Pence, a ticket that can stand up for the unborn, who can't stand up for themselves. You're starting to see a change in all these things."
The march is held each year close to the anniversary of the court's Roe v. Wade ruling that legalized abortion in 1973.
Trump has said Roe v. Wade should be overturned and has vowed to appoint an anti-abortion justice to fill the Supreme Court seat left vacant by the death of Justice Antonin Scalia last year.
He also has pledged to defund Planned Parenthood, which draws the ire of many Republicans because it provides abortions, along with other services.
Abortion rights supporters say cutting off funding for abortion providers will prevent poor women from getting other critical heath care and birth control that could prevent unwanted pregnancies.
The rally comes as the number of U.S. abortions has fallen to a record low. The Guttmacher Institute, a reproductive health and rights organization, said last week that it dropped below 1 million in 2013 for the first time since 1975.
Anti-abortion forces are often inspired by a religious conviction that life begins at conception and see abortion is murder.
Banner/thumbnail credit: Reuters