Tatyana Usmanova didn't believe a war between Russia and Ukraine would break out, so when she woke up to the news of Vladimir Putin ordering an attack in the early hours of Thursday, she thought it was just a bad dream.

"For some reason I woke up at 5:30 and decided to refresh Twitter," Usmanova, an opposition activist in Moscow, wrote on Facebook. "At first I thought I was just dreaming about waking up. I even walked around the room and touched things around me to make sure it's all real. "

Usmanova called the attack "a disgrace that will be forever with us now."

"It will become a huge trauma for the entire nation, which we will spend years coping with," she wrote. "I want to ask Ukrainians for forgiveness. We didn't vote for those who unleashed the war."

Moscow protests Ukraine invasion

Demonstrators march with a banner that reads: "Ukraine - Peace, Russia - Freedom," in Moscow on Thursday, Feb. 24, 2022, after Russia's attack on Ukraine. (AP Photo/Dmitry Serebryakov)

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Dozens of posts similar to Usmanova's came pouring in Thursday, condemning Moscow's most aggressive actions since the Soviet invasion of Afghanistan in 1979. Putin described the attack as a "special military operation" to protect civilians in eastern Ukraine from "genocide" – a false claim the U.S. had predicted he would make as a pretext for an invasion.

As sirens were blasting in Kyiv, Ukraine's capital, and large explosions were heard there and in other cities, Russians were signing open letters and online petitions demanding the Kremlin stop the violence, which Ukrainian forces reported had killed more than 40 soldiers and wounded dozens.

"Public opinion is in shock, people are in shock," political analyst Abbas Gallyamov told The Associated Press.

One petition, started by a prominent human rights advocate, Lev Ponomavyov, garnered over 150,000 signatures within several hours of being launched and 289,000 by the end of the day. More than 250 journalists put their names on an open letter decrying the aggression. Another one was signed by some 250 scientists, while 194 municipal council members in Moscow and other cities signed a third.

"I’m worried about the people very much, I’m worried to tears," said Zoya Vorobey, a resident of Korolyov, a town outside Moscow, told the AP, her voice cracking. "I’ve been watching television since this morning, every minute, to see if anything changes. Unfortunately, nothing (changes) so far."

Several Russian celebrities and public figures, including those working with state TV, spoke out against the attack as well. Yelena Kovalskaya, director of a state-funded Moscow theater, announced on Facebook she was quitting her job, saying, "It's impossible to work for a killer and get paid by him."

Pickets and protests broke out in several Russian cities, and calls to gather for a demonstration in the center of Moscow and St. Petersburg were making the rounds on social media in the morning.

"I know that right now many of you feel desperation, helplessness, shame over Vladimir Putin's attack on the friendly nation of Ukraine. But I urge you not to despair," human rights activist Marina Litvinovich said in a video statement on Facebook, calling on Russians to protest in their cities Thursday evening.

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"We, the Russian people, are against the war Putin has unleashed. We don't support this war, it is being waged not on our behalf," Litvinovich said.

But the authorities were having none of that.

In Moscow and other cities, they moved swiftly to crack down on critical voices. Litvinovich was detained outside her residence shortly after posting the call for protests. OVD-Info, a rights group that tracks political arrests, reported that 1,620 people in 52 Russian cities have been detained for protesting against the invasion, at least 872 of them in Moscow.

Russia's Investigative Committee issued a warning Thursday afternoon reminding Russians that unauthorized protests are against the law.

Roskomnadzor, state communications and media watchdog, demanded that Russian media use "information and data they get only from official Russian sources." Some media reported that employees of certain state-funded companies were instructed not to comment publicly on the events in Ukraine.

Human rights advocates warned of a new wave of repression on dissent.

Moscow protests Ukraine invasion

Police officers detain a demonstrator with a poster that reads: "I'm against the war," in Moscow on Thursday, Feb. 24, 2022. (AP Photo/Denis Kaminev)

"There will be new (criminal) cases involving subverters, spies, treason, prosecution for antiwar protests, there will be detentions of journalists and bloggers, those who authored critical posts on social media, bans on investigations of the situation in the army and so on," prominent human rights advocate Pavel Chikov wrote on Facebook.

"It is hard to say how big this new wave will be, given that everything has been suppressed already."

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Despite the pressure from the authorities, more than 1,000 people gathered in the center of Moscow Thursday evening, chanting "No to war!" as passing cars honked their horns.

Hundreds also took to the streets in St. Petersburg and dozens in Yekaterinburg.

"This is the most shameful and terrible day in my life. I even was not able to go to work. My country is an aggressor. I hate Putin. What else should be done to make people open their eyes?" Yekaterina Kuznetsova, a 40-year-old engineer who joined the demonstration in St. Petersburg, told the AP.

Russia's official line in the meantime remained intransigent. Speaker of the upper house of parliament, Valentina Matviyenko charged that those who spoke out against the attack were only caring about their "momentary problems."

State TV painted the attack in line with what Putin said in his televised address announcing it.

Russia 1 TV host Olga Skabeyeva called it an effort "to protect people in Donbas from a Nazi regime" and said it was "without exaggeration, a crucial junction in history."