Live Updates: Ukraine Agrees to Talks with Russia, as Putin Places Nuclear Forces on Alert

Many Russian protesters said they were dumbfounded by Mr. Putin’s decision to send troops and heavy weaponry into what many in Russia consider a “brotherly nation.” Millions of Russians have relatives or friends in Ukrainian cities. Many were raised in Ukraine and cherish childhood memories of it.

At the protests, many people said they came to express solidarity with the Ukrainian people and were confident that Mr. Putin’s decision will severely damage Russia.

Fyodor Gurov, for instance, said he never took part in protests before, but he was shocked when he read the news on Thursday that Russia attacked Ukraine, a country where his relatives live.

“I started feeling shame that I live in Russia,” Mr. Gurov, 22, said, speaking on the phone from a police van, where he was being detained.

On Sunday, Mr. Gurov said he came to stand in front of the Russian Foreign Affairs building in central Moscow with a poster saying “No to war!” Shortly after he came there, the police detained him, threatening to brake his hand. A flight attendant, Mr. Gurov is also afraid of losing his job after European countries blocked airspace to Russian flights.

In Moscow, crowds of people moved around the city center, chanting “No to war!” To make it harder for police to detain them, they tried not to concentrate in a single place. Still, the police rounded up more than 1,100 people in the Russian capital alone and more than 1,100 in other Russian cities according to OVD Info, a rights group that tracks arrests at demonstrations in Russia.

Apart from arresting people at demonstrations, Russian authorities also said they would increase pressure in other spheres. Government employees who signed letters and petitions against the war, for instance, were threatened with dismissal.

The Russian prosecutor general’s office warned Russians on Sunday that the provision of loosely defined “assistance to a foreign organization or their representatives in activities directed against the security of Russia” can be qualified as high treason, punishable by up to 20 years behind bars. Russia’s communications watchdog announced on Friday it would partially limit access to Facebook as retaliation for restricting some pro-Kremlin media accounts.

On Sunday, many Russians also came to a bridge opposite the Kremlin to lay flowers to the spot where Boris Nemtsov, a prominent Russian opposition politician, was brutally shot do death seven years ago. Throughout his political career, Mr. Nemtsov spoke against any form of Russian aggression against Ukraine.

Some people, who took part in the initial wave of protests on Thursday, could not go out again to avoid committing the same offense twice. Aleksei Kudasov, for instance, was arrested on Thursday and later released, so he decided not to take the risk, but said he was ready “to do everything to for this nightmare to stop.”

Mr. Putin’s decision will bring “nothing but grief to both sides of the conflict,” he said, adding that he would donate to rights organizations and will help spread truthful information about the conflict.

“People should not spend their nights in the metro for the president of another country to move tin soldiers on a map,” said Mr. Kudasov, 31, a copywriter. “Many of us have relatives and friends in Ukraine — to attack such a neighbor is an absolutely savage decision.”