The annual "March for Life" rally was due to begin at noon (1700 GMT) on the sprawling National Mall and take protesters past the Capitol, the seat of the US legislature, to the Supreme Court, where on January 22, 1973, the country's highest justices voted seven to two that a state law in Texas that banned abortion, except to save a woman's life, was unconstitutional.

The Supreme Court decision, enshrined as "Roe v. Wade" for the key figures in the case, gave the United States some of the least restrictive abortion laws in the world and galvanized religious opposition groups into action against it.

On the eve of the march, anti-abortion activists crowded into the Shrine of the Immaculate Conception at Catholic University in Washington to pray for abortion to be banned, officials at the university said.

While local media reported that thousands had attended the all-night vigil, university officials were still counting heads on Tuesday morning, spokeswoman Jackie Hayes said.

More than 1,000 "pro-life" advocates from around the United States spent the night sleeping on floors at the university ahead of the march, the school's website said.

The Reverend David O'Connell, president of Catholic University, attended a breakfast "with President George W. Bush and pro-life leaders" at the White House Tuesday morning, the website added.

Norma McCorvey -- originally given the pseudonym "Jane Roe" for the landmark case -- was due to hold an anti-abortion news conference alongside Republican presidential candidate Ron Paul in Washington ahead of the march.

McCorvey was first brought before a court in Dallas, Texas, in 1970 by two women lawyers seeking the right for the then-homeless, 22-year-old single mother to terminate her third pregnancy.

The defendant was Henry Wade, the district attorney representing the state of Texas.

The Texas court's ruling favored "Roe," but it still did not strike down Texas's state laws banning abortion.

So that by the time the case came to the Supreme Court and its decision in favor of universal abortion rights nationwide was handed down, McCorvey had already had her third child, which she put up for adoption.

Years later, she converted to Catholicism and became an anti-abortion activist.

"I believe that I was used and abused by the court system in America. Instead of helping women in Roe v. Wade, I brought destruction to me and millions of women throughout the nation," McCorvey told lawmakers in 2005.

Roe versus Wade is seen as increasingly vulnerable to being overturned, as the court has turned more conservative with recent appointments and it requires only five of the nine Supreme Court judges to vote in favor of changing the decision to permit the 50 states to set their own abortion bans.

In April, the high court took the first step in rolling back abortion rights in more than a generation by banning a controversial late-term termination procedure.

Public opinion has remained fairly constant over the years with some 25 percent of Americans believing abortion should be available on demand, 25 percent saying that it should be totally banned, an 50 percent holding that it should be legal but restricted.