Thousands of abortion opponents shouldering signs with slogans such as "Peace Begins in the Womb" marched in protest of the 33-year-old Roe v. Wade decision, while abortion rights supporters along the march route waved clothes hangers and shouted "Bigots go home."

The dueling protests — marking Sunday's anniversary of the Supreme Court decision — reflected the growing tension at a time the makeup of the high court is about to change with Justice Sandra Day O'Connor's retirement.

"It's a crucial time," said abortion rights supporter Carol Norris, 43, who joined the counter-protest in San Francisco Saturday. "We have (Judge Samuel) Alito poised to be on the Supreme Court, and he's clearly an anti-choice person."

On the other side, college student Laura Arnold, 20, of Pleasanton, Calif., marched with her mother opposing abortion, saying: "We're here to stand up for the babies that don't have a voice."

"I know so many girls who did it and they are hurting every day of their life," Arnold said.

The Supreme Court handed down its decision in Roe vs. Wade on Jan. 22, 1973, and abortion has been legal in the United States ever since. But efforts to restrict or outlaw the procedure have been just as enduring; 34 states have passed laws requiring parents either to be notified or to give consent when their underage daughters seek abortions.

"Abortion rights have been slowly whittled away while we haven't even been looking," said Kitty Striker, 22, who decorated her hair with small coat hanger replicas for the counter-protest. "That's what's so shocking and so scary to me."

Many abortion opponents said they were heartened by President Bush's choice of Alito to replace O'Connor, a moderate who was often the court's swing vote.

Alito's refusal during his confirmation hearings to agree with assertions by Democrats that Roe v. Wade was "settled law" upset abortion rights activists.

The largest abortion demonstration was expected Monday in Washington, D.C., where anti-abortion activists planned to converge on the mall to hear speakers supporting their cause and march on the Congress and Supreme Court.

In Michigan, a group pastors and ministry leaders used the anniversary Sunday to launch a new anti-abortion organization, Michigan Chooses Life. One goal is to support efforts to get a measure on the 2006 ballot that would change the state constitution to legally define a person as existing at the moment of conception. The American Civil Liberties Union of Michigan has said that even if the measure did succeed, it would be challenged in court.

At the San Francisco protest, Archpriest Michael Regan of St. Michael's Orthodox Church in Concord, Calif., said it was important to show how mixed public opinion is on abortion, even in the liberal San Francisco area.

"You do get the impression that there isn't anyone here for the right to life, but look around," he said.