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

During the protests, many people said they had come to express their solidarity with the Ukrainian people and were convinced that Mr. Putin’s decision would seriously damage Russia.

Fyodor Gurov, for example, said he had never participated in protests before, but was shocked when he learned on Thursday that Russia had attacked Ukraine, a country where his relatives live.

“I started to feel ashamed of living in Russia,” said Mr Gurov, 22, speaking on the phone from a police van, where he was being held.

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

In Moscow, crowds of people moved through the city center chanting “No to war! To make it harder for the police to detain them, they tried not to concentrate in one place. Still, police have arrested 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 during protests in Russia.

Besides arresting people during protests, Russian authorities have also said they will increase pressure in other areas. Government employees who signed letters and petitions against the war, for example, were threatened with dismissal.

Russia’s attorney general’s office on Sunday warned Russians that providing “assistance to a foreign organization or its representatives in activities directed against the security of Russia” can amount to high treason, which is punishable up to 20 years in prison. Russia’s communications watchdog said on Friday it would partially restrict access to Facebook in retaliation for restricting some pro-Kremlin media accounts.

Many Russians also came to a bridge opposite the Kremlin on Sunday to lay flowers at the spot where Boris Nemtsov, a prominent Russian opposition politician, was brutally shot seven years ago. Throughout his political career, Mr. Nemtsov has spoken out against any form of Russian aggression against Ukraine.

Some people, who had taken part in the first wave of protests on Thursday, were unable to come out again to avoid committing the same crime twice. Aleksei Kudasov, for example, was arrested on Thursday and then released. He therefore decided not to take the risk, but said he was ready to “do everything to make this nightmare stop”.

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

“People shouldn’t be spending their nights on the subway for the president of another country to move toy soldiers on a map,” said Mr. Kudasov, 31, the editor. “Many of us have relatives and friends in Ukraine – attacking such a neighbor is an absolutely savage decision.”