There were emotional scenes as hundreds gathered at a Trafalgar Square vigil marking the third anniversary of the Russian invasion on Monday night.
More than 1,000 people filled the central London square in a gathering also attended by MPs.
The event heard an attack on the US for wavering on its support for Ukraine from the president of the Ukrainian World Congress.
In a pre-recorded speech, streamed on to a large screen, Paul Grod warned: “If Ukraine falls, Europe is next – this is not just Ukraine’s fight, this is a fight for the future of freedom.”
The president of the international non-governmental organisation (NGO), which represents Ukrainian communities in more than 60 countries, described “the horrors, the systemic kidnapping, the torture, and the abuse of Ukraine’s most vulnerable, its children”.
“No tyrant can be allowed to redraw borders by force, no ally should pressure Ukraine to surrender,” he said.
“This is not a conflict where deals can be made, you cannot negotiate with someone who is seeking to destroy you.
“Today we face a troubling moment – for the first time since Russia’s full-scale invasion, the United States has begun to waver.
“This week the US refused to co-sponsor a United Nations resolution condemning Russia’s war – this is unacceptable. Now more than ever the world must stand united, Ukraine’s fight is the world’s fight.”
Mr Grod emphasised that Ukraine is a democracy and “every major opposition party” has agreed to postpone elections until the end of the war.
He said Russian President Vladimir Putin “has ruled for 25 years by jailing and murdering his opponents”, adding: “He is not a leader he’s a dictator.”
Last week US President Donald Trump described Ukraine’s President Volodymyr Zelensky as “a dictator without elections” in a tirade on social media.
Mr Grod said: “We must be absolutely clear, Russia will not stop unless it is defeated.
“Putin will never stop, his goal is not just Ukrainian land, he wants to erase Ukraine as a nation.”
He added that “de-Nazification is a mask for genocide”.
Moscow said the “de-Nazification” of Ukraine was one of the key goals of the invasion.
Organisers of the Trafalgar Square gathering included the Ukrainian Catholic Eparchy of the Holy Family of London, Support Ukraine, the British-Ukrainian Aid charity, Ukrainian Autocephalous Orthodox Church and St Mary’s Ukrainian School.
Earlier on Monday shadow foreign secretary Dame Priti Patel joined other MPs at a prayer service at a Ukrainian church in central London.
Hymns, prayers and a commemorative candle lighting were delivered during the hour-long interfaith service at the Ukrainian Catholic Cathedral in Westminster.
Dame Priti was joined by migration and citizenship minister Seema Malhotra and the Ukrainian ambassador to the UK, General Valerii Zaluzhnyi.
