They acknowledged the plight of the eastern European nation as fears grow Russia is planning a greater onslaught.
The heir to the UK throne said he and his wife had been moved by the “bravery, generosity and fortitude” of the Ukrainians in the face of the military action by Russian forces.
Cathedral staff reported that Ukrainian men have been seeking blessings before travelling back to their homeland to join the fight against Mr Putin’s army.
Camilla was left close to tears by a mournful rendition of the song Oy, u Luzi Chervona Kalyna, to welcome the couple.
Men, women and children performed the song, which was sung by Ukrainian riflemen during the First World War.
For the second day, Prince Charles spoke out about the conflict in Ukraine.
“I must say my wife and I have been deeply moved by everything we’ve heard today during our visit and above all by the extraordinary bravery, generosity and fortitude of the Ukrainian community in the face of such truly terrible aggression,” he said.
On Tuesday, during a visit to Southend-on-Sea, Essex, he said the values of democracy were under attack in Ukraine in the “most unconscionable way”.
It is understood the heir to the throne had approached the Ukrainian community offering his help and he brought representatives from five humanitarian organisations he is connected with to offer practical support.
The couple were also joined by the Ukrainian ambassador, Vadym Prystaiko, and his wife, Inna, who earlier received a rare tribute from MPs, a standing ovation in the House of Commons when he watched Prime Minister’s Questions from the gallery.
Ms Prystaiko was in tears as the duchess held back her own at the start of the event and when the royal couple left Camilla hugged the diplomat’s wife and said they would pray for her.
Charles’ royal event followed a visit by the Prime Minister to the cathedral on Sunday and emphasised the UK’s position of public support for Ukraine.
Mr Prystaiko said he was “touched” by the duchess hugging his wife and said he would convey the message this symbolised back to his homeland.
He was asked about Charles’ comments and disagreed with the suggestion they were political: “It’s not political any more. We’re past political, we’re in survival mode.
“We’re now, as the prince mentioned, trying to find a way how a nation of 40 million people can survive the aggressor.”
“We hope and pray we will be able to stop this, the question is how without real support from Nato, from the UK, from the United States … he’s still bombarding our cities,” the diplomat said.
Ruslana Bigur has lived in Britain for three years.
“He said the situation was unbelievable in the 21st century,” she said after meeting Prince Charles.
Ms Bigur, an accountant, said her parents in Ukraine had refused to move to safety.
“They want to stay because it’s their land and it’s home to them,” she said.
At the end of the visit, Charles spoke a few words of Ukrainian saying the phrase well-known in the eastern European country: “Glory to Ukraine” and received the response “Glory to the heroes” from the guests.
The couple lit candles in the cathedral. Each left a single sunflower, the national flower of Ukraine.
Bishop Kenneth Nowakowski, of the Ukrainian Greek Catholic Church, hosted the visit.
He said Charles had genuine feelings of shock about events in Ukraine.
“From the moment we walked into the cathedral, one of his first words was how shocked he was to know what was happening and again expressed his solidarity and compassion,” he said.
“I felt he was talking to me, not just to Bishop Kenneth, but to me as a person whose friends and colleagues are suffering in Ukraine and thereby also expressing that to the wider Ukrainian community in the United Kingdom.”
Of the blessing given to men destined to fight, Bishop Kenneth said: “We certainly don’t pray that they kill anybody.
“We want to bless them and assure them of our prayers, and if they have to defend themselves, they have to defend themselves.”