Demonstrators denounced Russian President Vladimir Putin as a “delusional lunatic” and a “bully” as hundreds gathered in central London to protest his invasion of Ukraine.
Some members of the crowd, including expats from Russia, Poland and Lithuania, wept as they voiced support for people affected by the war.
Campaigners gathered outside the BBC’s Broadcasting House to wave signs saying “Russian troops out” and “No Nato Expansion”.
The march was held by the Stop the War Coalition, as well as the Campaign for Nuclear Disarmament (CND), the No to Nato network, and CODEPINK.
Stop the War has caused controversy in recent months for its Nato-critical stance.
It has described itself as opposed to the British Government’s “aggressive posturing” and Nato’s “eastward expansion”.
But it has been repeatedly accused of harboring anti-Western sentiments.
One male protester, who did not give his name, said Nato countries were “provok[ing] Putin”.
However, some marchers voiced differing opinions on the bloc.
Monika Lichomska, a Polish warehouse worker, said she was there to show her support for Ukrainians and believes Nato support can offer security.
“In Poland, we are secure because of Nato. But if we’re not stopping (Russia) now they will come after our country,” the 37-year-old, who described Mr Putin as a “delusional lunatic”, said.
Zoja, an NHS worker whose parents are Russian, wiped away tears as she said she felt the “need to apologise” for Mr Putin’s actions.
“Russia does not represent Russian people. We are not standing with him, we are standing against him,” she said.
“He declared war in our name but I didn’t ask him to do it.
“Continue fighting and I believe we will win. Russian soldiers have nothing to fight for.”
Jane Calvert, an NHS worker, 61, said Britain should not “bait a bear” and should instead pursue de-escalation.
“(Mr Putin) is not a leader. He’s a bully and a dictator,” she said.
Andrew McCann, another NHS worker, said the protest appeared to have “hijacked” by those with other agendas.
“I’ve seen people here who want a Socialist Worker’s Party and ones that don’t want a Socialist Worker’s Party and that route can tend to hijack these things.
“They’re entitled to be here but there seems to be a bit too much nowadays of, ‘You’re wrong and I’m right.’
“Can’t we have a common argument against violence towards innocent people?”
Mr McCann, who supports Stop the War and CND, said he is concerned about young people online being exposed to anti-Ukrainian propaganda.
“There’s a big push to make people believe that, in Ukraine, the whole country is racist, for example, and we need to do more to make people question that,” he said.
Protesters chanted “Stop the war” and “Russian troops out now” as they began their march through the center of the capital.
Speakers were due to address the crowd at Trafalgar Square on Sunday afternoon.