The Ukrainian people have "suffered unimaginably", King Charles has said in a message marking the first anniversary of Russia's invasion.
Rishi Sunak and his wife Akshata Murthy weres joined by Ukraine's ambassador to the UK for the minute's silence outside 10 Downing Street
He also praised their "remarkable courage and resilience" after thousands have been killed and injured.
A minute's silence was held across the UK at 11:00 GMT.
Rishi Sunak later urged allies at a G7 meeting to provide Ukraine with long-term military and security assurances to "send a strong message" to Russia.
Ukrainian troops who are training in the UK joined the prime minister, his wife Akshata Murthy, and Kyiv's ambassador to Britain, Vadym Prystaiko, for the minute's silence observed outside No 10 Downing Street.
The Ukrainian national anthem was sung to mark the end of the silence.
In his message, the King said "the people of Ukraine have suffered unimaginably from an unprovoked full-scale attack on their nation. They have shown truly remarkable courage and resilience in the face of such human tragedy".
He said: "The world has watched in horror at all the unnecessary suffering inflicted upon Ukrainians, many of whom I have had the great pleasure of meeting."
The King, who met Ukraine President Volodymyr Zelensky at Buckingham Palace this month, added: "I can only hope the outpouring of solidarity from across the globe may bring not only practical aid, but also strength from the knowledge that, together, we stand united."
At a vigil on Thursday evening, a crowd listened to an emotional reading of the Ukrainian poem Take Only What Is Most Important by actress Dame Helen Mirren - who was visibly moved to tears. And Defence Secretary Ben Wallace paid tribute to Ukrainian soldiers as the "bravest of the brave".
The conflict, which began when Russia invaded on 24 February 2022, has seen at least 100,000 of each side's soldiers killed or injured, according to the US military.
Thousands of civilians have also died, with more than 13 million people made refugees or displaced within Ukraine.
Rita and her four children were among those who fled in the early stages of the conflict and are now living in the UK with her British partner, Andy.
She told BBC Two's Newsnight programme her heart was "aching" from seeing how parts of Ukraine had changed after 12 months of conflict.
"The country is in pain," she said. "I know how my country is and how it can be, I know how beautiful it is. Now it's different [but] it can come back to that beautiful place."
The Archbishop of Canterbury called for peace between Russia and Ukraine as he reflected on the anniversary.
Speaking on BBC Radio 4's Thought of the Day segment, Justin Welby said: "There must be a future with a just and stable peace - a free and secure Ukraine - and the beginning of a generation's long process of healing and reconciliation."
The British ambassador to Ukraine, Dame Melinda Simmons, has recalled how the outbreak of the war last February was "such a traumatic time".
Dame Melinda told BBC Radio 4's Woman's Hour programme her role became different and stopped being a job and became "a life because war isn't just a five-day thing".
Meanwhile, the Foreign, Commonwealth and Development Office has announced fresh export bans on goods that could be used by the Russian military.
Foreign Secretary James Cleverly said the UK has ramped up sanctions on more products, including aircraft parts, radio equipment and electronic components.
Bosses at Russia's two largest defence companies and four banks will also face sanctions.
However, Nato Secretary General Jens Stoltenberg said Ukraine's allies still could do more.
At a press conference on Friday, he said that the wave of sanctions imposed by Western nations "do not seem to have dented the Kremlin's ability or desire to wage war".
During a recent tour of Europe, President Zelensky increased his calls for Western nations to supply modern fighter jets.
The UK is to start training Ukrainian forces to fly Nato-standard aircraft. But like other Western nations, it has so far not supplied jets, but said it remains a long-term option.
Defence Secretary Ben Wallace said the UK would be "very happy" to supply fighter jets to eastern European allies so they could release their Soviet-era planes to Ukraine. He said they were already being used by Kyiv and it would be a faster way of boosting Ukraine defences than suppling British Typhoon jets.
During a virtual meeting of leaders from the G7 group of advanced economies, Mr Sunak said that an acceleration in support for Ukraine is "what it will take to shift Putin's mindset".
He made the argument for supplying Ukraine with "longer-range weapons" to disrupt Russia's ability to target Ukraine's infrastructure, something to which he committed the UK earlier this month.
He said: "Instead of an incremental approach, we need to move faster on artillery, armour, and air defence."
Other senior UK politicians have sent messages to Ukraine on the anniversary of the war:
* Labour's Sir Keir Starmer called for the UK to "double down" on support for Ukraine
* Former Prime Minister Boris Johnson, who was in office when Russia invaded, repeated his call to give Ukrainian President Volodymyr Zelensky weapons "to finish the job"
* Liberal Democrat leader Sir Ed Davey paid tribute to the "amazing acts of heroism in Ukraine" and said the UK would "stand in solidarity with Ukraine until they achieve victory"
* SNP leader and Scottish First Minister Nicola Sturgeon wished a "speedy victory for Ukraine" in a letter to Ukrainians in Scotland to mark the anniversary