The march represented the most prominent expression of public anger thus far against the heavily criticized measure, which took effect Jan. 1 after being passed by the State Duma with overwhelming support and signed into law by President Vladimir Putin.
Two columns of people holding banners with images of deputies and senators who supported the bill and of Putin with the word "Shame!" written over them in red marched down both sides of Strastnoi Bulvar toward Prospekt Akademika Sakharova, an oft-used site for opposition rallies.
The turnout appeared to be much larger than that for other protests of recent months. Police said around 9,000 to 9,500 people took part, while opposition leader Sergei Udaltsov said more than 50,000 people were in attendance. Some participants estimated that there were more than 100,000 people there, outstripping protests held in December 2011 following disputed parliamentary elections.
Most participants interviewed said they didn't think the bill would be reconsidered by parliament but that they felt compelled to attend the protest anyway.
"I consider it my duty to come today, as I am a young mother myself," said Svetlana Ardayeva, a 27-year-old housewife. "Children's rights are sacred, and the bill violates these rights dramatically."
The two columns came together at Prospekt Akademika Sakharova, where the march concluded and participants threw the portraits of politicians into a large garbage container.
"The march was emotional, with tough slogans. This kind of beginning to the year is great," said Udaltsov, who led one of the columns and said the march represented the revival of the weakening protest movement.
Protests against the law also took place in other cities and towns across Russia on Sunday, including St. Petersburg, Rostov-on-Don and Barnaul.
The demonstrations came amid confusion over when the ban will take effect and to which potential U.S. adoptive parents it will apply.
A bilateral adoptions agreement that went into effect last year stipulates that a country wishing to exit the deal must warn the other signee a year in advance, during which time the agreement remains in force.
But Putin's spokesman Dmitry Peskov said Thursday that most U.S. families planning to adopt Russian children will not be able to do so.
Peskov told Dozhd television that only if the adoption procedure had been completed by a court before Jan. 1 would the U.S. parents be allowed to take the child.
This means that U.S. families that had started legal procedures to adopt a Russian child before Jan. 1 but hadn't received a court ruling granting them custody rights before the law took effect will not be able to adopt the child.
"This is children we're talking about, and children are, always have and always will be treated with a great deal of attention," Peskov said. "No one will take a formal approach on this. But there is the law, and this law says that adoptions are banned."
The Foreign Ministry informed the United States of its intent to terminate the agreement in early January, Peskov said.
The adoptions ban was part of Russia's response to the Magnitsky Act, passed by the U.S. Congress in December, which imposes travel restrictions and economic sanctions on Russian officials implicated in human rights abuses.
The adoptions ban has evoked a storm of criticism from Russian bloggers since it was proposed by United Russia lawmakers in December. Tens of thousands of Russians signed a petition calling on the U.S. Senate to put restrictions on Russian lawmakers who supported the ban and another petition urging the Duma to dissolve.
Last month, around 7,000 U.S. citizens also signed a letter asking Putin not to enact the law.
Many bloggers have expressed particular outrage over the fact that the law, they believe, deprives disabled orphans of a chance to receive good medical care and live a productive life abroad.
Duma Deputy Robert Shlegel of United Russia submitted an amendment to the law in late December that would have made an exception for disabled children, but he recalled it on Friday.
The amendment "cannot be passed in its current form" and work on it will continue, Shlegel tweeted.
Duma Deputy Dmitry Gudkov of A Just Russia, who voted against the ban, called the bill "hysteria, not an answer to the Magnitsky list."
"We are conducting our own investigation. Most deputies who supported the bill may be on the Magnitsky list themselves," he said before Sunday's march.
He also said a petition against the ban organized by opposition newspaper Novaya Gazeta that has gathered more than 100,000 signatures would be considered by the Duma on Monday.
The Kremlin human rights council sent Putin an evaluation of the bill that said it contradicted the Constitution because federal authorities were required by law to have consulted regional authorities before its passage.
On Sunday, Peskov told Interfax that Putin had been informed about the Moscow protest march. "He is certainly aware of it," he said.
Commenting on the protest, Peskov said that people's concern about the lives of orphans "certainly deserves attention and can be understood" but that calls to dissolve the State Duma were "non-constructive."
Senior Duma Deputy Yekaterina Lakhova of United Russia condemned the protesters, saying they rallied not to protect Russian orphans but to support U.S. businesses.
Lakhova told RIA-Novosti that she had studied the business of U.S. adoptions of Russian children and that U.S. agencies lost large sums of money because of the ban on adoptions.
She labeled the protest "purely political" and hinted that it was masterminded by an unidentified entity.
The march was attended by leaders of many of the political groups that have led the non-parliamentary opposition over the last 13 months, including Ilya Yashin of the Solidarity movement and Boris Nemtsov, Vladimir Ryzhkov and Mikhail Kasyanov of the People's Freedom Party — Republican Party of Russia.
People took up various chants as they marched along the hills of the Boulevard Ring, shouting "Putin to the Magnitsky list," "Withdraw the scoundrels' bill," and "Shame!"
Another popular slogan was "Down with President Herod," referring to the ancient Roman leader who in the Bible orders the killing of all children under the age of two in and around Bethlehem.
"We feel pity for the children and came here out of a sense of human dignity. Putin did exactly the same thing as Herod," said pensioner Zhanna Polyakova, 74.