MOSCOW — President Vladimir V. Putin on Friday signed into law a ban on adoptions of Russian children by American citizens, apparently blocking the departure from Russia of hundreds of orphans who had already been told that they would soon go home with new parents.Among the children whose lives were caught up in uncertainty by the ban was a 3-year-old girl with H.I.V. Her adoption by a couple from the Rocky Mountain West was approved by a judge on Thanksgiving Day but still required a 30-day waiting period, followed by numerous bureaucratic steps that cannot be completed before the ban will become effective on Tuesday.“I’m really, really stressed out,” said the adoptive mother, who asked not to be identified to protect the privacy of the girl she still expected to bring home.She said she was filled with second thoughts about whether they had done everything they could to assure the adoption. “Why weren’t we on a plane as soon as there was even a mention of the ban?” she said.The adoption ban, which was included in a broader law retaliating against the United States for an effort to punish Russian human rights violators, has opened a deep and emotional schism at the highest levels of the government and more broadly throughout Russian society. It has also dealt a severe blow to the country’s already strained diplomatic relationship with the United States.The Kremlin’s announcement that Mr. Putin had signed the law set off fierce reactions and some immediate second-guessing, even by some of the president’s political allies.Robert Schlegel, a lawmaker from the majority United Russia party, which championed the adoption ban in the lower house of Parliament, posted on Twitter that he had proposed an amendment that would create an exception to the ban for children with disabilities. Critics asked why he had not done so before the measure was approved.A number of commentators, including Vladimir Varfolomeyev, a well-known host on the radio station Ekho Moskvy, compared Mr. Putin to the biblical King Herod, noting that the adoption ban was signed on the same day that the Orthodox Church commemorates the Massacre of the Innocents, when Herod ordered the killing of infants in Bethlehem.There was also sharp reaction in Washington, where some officials seemed to have been holding back in the vain hope that Mr. Putin would veto the ban.In a statement, Senator John McCain, Republican of Arizona, called the ban “shameful and appalling.”“The effects of this legislation are cruel and malicious,” Mr. McCain said, adding, “To punish innocent babies and children over a political disagreement between our governments is a new low, even for Putin’s Russia.”Mr. McCain was a leading supporter of the American law targeting human rights violators in Russia. That law is named for Sergei L. Magnitsky, a Russian lawyer who tried to expose government tax fraud but was arrested and died in prison in 2009 after purportedly being denied proper medical care.In a judicial corollary to the dispute over the Magnitsky bill, a Moscow judge on Friday acquitted the only official to be tried on charges related to Mr. Magnitsky’s death.The official, Dmitry Kratov, the former head of the medical service at Butyrka Prison, where Mr. Magnitsky had been held, had been charged with negligence for refusing repeated requests for treatment of a life-threatening illness.Charges against another doctor were dismissed this year, and in closing arguments on Monday, the prosecutor did an about-face and urged the acquittal of Dr. Kratov, saying there was no evidence that he was responsible for Mr. Magnitsky’s death.That reversal came four days after Mr. Putin, at his annual news conference, told journalists that Mr. Magnitsky had died of a heart attack, and waved off criticism by noting that prisoners die in jails all the time, including in the United States.Dr. Kratov was the only person on a list of 60 Russian officials implicated in the Magnitsky case by the U.S. Helsinki Commission to be brought to trial in Russia.Nikolai Gorokhov, a lawyer representing the Magnitsky family, said that Dr. Kratov had signed documents refusing Mr. Magnitsky’s request to be moved to an infirmary, and that he had been aware of a diagnosis of pancreatitis and gallstones five days before Mr. Magnitsky’s death.The Magnitsky law bars individuals like Dr. Kratov, who have been accused of human rights abuses, from traveling to the United States or from owning property or other assets here.Russian policy makers were vexed in trying to come up with a reciprocal response, in large part because Americans typically do not vacation, own homes or maintain assets in Russia. Wealthy Russians, on the other hand, have been involved in some of the biggest real estate deals in America in recent years, and often travel to the United States.The initial legislative response in Russia focused on sanctions similar to those in the Magnitsky law on American judges and others accused of violating the rights of adopted Russian children in the United States. The law was named for Dmitri Yakovlev, a toddler who died of heatstroke in Virginia in 2008 after his adoptive father left him in a parked car for nine hours. The father was acquitted of manslaughter by a judge who ruled the death an accident.Such cases have generated outrage here, which spilled over in 2010 when a Tennessee woman who was the adoptive mother of a 7-year-old boy sent him alone on a flight back to Russia with a note saying she could no longer handle him.Along with the Yakovlev law, Mr. Putin signed a presidential decree ordering that the government take steps to encourage additional adoptions by Russians, and to improve health care for orphans.Child welfare advocates, however, have mocked the decree as lip service in a country where more than 650,000 children live in foster care or orphanages, of whom about 120,000 are eligible for adoption. Many children in orphanages are sick or disabled, and most are unlikely to ever find permanent homes.Critics of the ban say there are not enough Russians willing to adopt healthy children, let alone those with special needs.Heather Whaley and her husband, Aaron, from Frederick, Md., were willing to do so and have been matched with a 4-year-old girl with several developmental delays who is in an orphanage in Vladivostok in the Russian Far East. They have gone through exhaustive screening, and have been scraping together money to visit the girl, who is named Regina.But now they say they do not know what to expect.Ms. Whaley, 31, a therapist for special-needs children, and her husband, 28, an engineer, said devout Christianity drove them to adopt, along with Ms. Whaley’s happy personal experience when her parents adopted two girls, giving her two new sisters.Regina, according to medical reports, is tiny for her age, and her speech is delayed. Russian families have not offered to take her. “With my training, I firmly believe that I can help her,” Ms. Whaley said.The mother from the American West said she had visited her daughter twice this year, in July and in November. The little girl has blond hair and big blue eyes and is so active that the orphanage director has said, “I think there’s a boy in that little girl.”But because she is H.I.V.-positive and already 3 years old, her chances of being adopted are slim.“I don’t think she is desirable to anybody in Russia at this stage of the game,” said the mother, who still plans to travel to Russia next month. “She would grow up in an orphanage for her entire life and be turned out when she comes of age.”