Protesters started at a statue of St Volodymyr, a national saint of Ukraine, in west London and marched towards the Russian embassy, waving Ukrainian flags and signs of support.
One sign read “Ukraine defends peace for the entire Europe”, while another said “If Ukraine falls, war will come to your house”.
The three-year long war was sparked by the Russian invasion of Ukraine on Feb 24, 2022.
Britain has been a key backer of Ukraine since, sanctioning Moscow, providing financial and military support to Kyiv and opening its door to over 250,000 Ukrainian refugees.
“I’m not Ukrainian but I recognise the great danger they are in,” 68-year-old Briton Martin Vincent told AFP.
“We cannot abandon them, it’s a duty for the UK to stand up with Ukraine,” the protester added.
Among the Ukrainians at the rally was Maryna Tsymbaliuk, 21, a student living in London.
“The world should know that the last word will be from Ukrainians by Ukrainians,” Tsymbaliuk said.
“Nothing can be made without us at the table,” she added, referencing the sidelining of Kyiv by the US in recent talks with Russia on the future of the war.
“It’s more important than ever to stand up and to resist and to talk about it… and Europe has to be more united than ever.”
Nataliya, a university student who did not want to share her last name for security reasons, is from Ukraine as well.
“I feel so homesick and so vulnerable right now. I don’t know If I’ll be able to come back to my country,” said Nataliya, wearing a floral crown in the yellow and blue of the Ukrainian flag.
“What’s next? Uncertainty and uncertainty,” she added.
Stella Robinson, 27, was “afraid of what might happen next”. “This is not only Ukraine, this is Europe.”
“We can’t turn a blind eye on the war just because Trump wants peace. But what kind of a peace? Frankly, it’s terrifying,” said the law student.
In a call with Ukrainian President Volodymyr Zelensky on Saturday, British PM Keir Starmer reiterated “UK’s ironclad support for Ukraine and commitment to securing a just and enduring peace”, according to a Downing Street spokesperson.
British public support for Ukraine is strong too, with 67% saying they both want Ukraine to win the war and care a “great deal or fair amount” that it does so, according to a YouGov poll from last week.
And eight in ten Britons said it is “unacceptable” for Ukraine not to be included in negotiations on the conflict, per the poll.
Thousands of Ukrainian civilians have been killed since the start of the war, although the exact toll is unclear.