Tens of thousands of anti-abortion activists are set to take part in Monday's annual "March on Washington" to mark the 34th anniversary of a Supreme Court decision that legalized abortion.
Similar marches and rallies are also taking place in cities and towns across the United States.
In Washington, D.C., abortion opponents will march past the U.S. Capitol and end outside the Supreme Court.
President Bush is scheduled to address the demonstrators by telephone. Senator Sam Brownback, a socially-conservative Republican who announced Saturday he is running for president in 2008, is participating in the March for Life events.
Activists on the other side of the issue who support abortion rights will also be holding smaller rallies in Washington and other cities.
The "Roe versus Wade" Supreme Court decision legalizing abortion was handed down on January 22, 1973.