PIERRE, South Dakota (Reuters) - South Dakota's governor signed into law on Tuesday the longest abortion waiting period in the nation at 72 hours, and opponents immediately promised a legal challenge to stop it from going into effect.

The law signed by Republican Gov. Dennis Daugaard also requires a woman to submit to counseling to ensure her decision to have an abortion is "voluntary, uncoerced, and informed."

The new law is one of many abortion curbs being pushed by conservative lawmakers in dozens of states this year. Other proposals include bans on late-term abortions and requirements that providers offer women sonograms of their fetuses.

"I think everyone agrees with the goal of reducing abortion by encouraging consideration of other alternatives," Daugaard said in a statement. "I hope that women who are considering an abortion will use this three-day period to make good choices."

No other U.S. state has a waiting period longer than 24 hours, according to the Guttmacher Institute, which supports reproductive rights. The South Dakota law also is the first in the nation to require a counseling session at a center whose mission is to encourage women to continue their pregnancies.

Planned Parenthood, which operates the only health center in South Dakota that provides abortions, said on Tuesday it would file a lawsuit to stop the law. Supporters of the legislation have pledged to raise private funds to finance a defense of the law that is scheduled to take effect on July 1.

South Dakota has been at the center of some of the most bitter recent fights over abortion, which was legalized in 1973 by the U.S. Supreme Court in Roe v. Wade.

State lawmakers passed laws in 2006 and 2008 to ban most abortions unless they were necessary to save a woman's life. Voters later overturned both bans.

"We know that women reflect, talk with friends and family, and consult with pastors and their doctors before making this difficult decision," said Sarah Stoesz, president and CEO of Planned Parenthood Minnesota, North Dakota, South Dakota.

Alisha Sedor, executive director of NARAL Pro-Choice South Dakota, said the bill was an "egregious" invasion of the doctor-patient relationship.

South Dakota lawmakers set aside another bill earlier this year that sponsors said would protect pregnant women from attack, but critics said could have legalized the killing of abortion providers in the state.

(Reporting by Michael Avok in Pierre, additional reporting and writing by David Bailey in Minneapolis; Editing By Greg McCune)