During his six hours in Ukraine, the prime minister called for a "complete reversal" of Russia's annexation of Crimea and suggested Russia should be booted out of the Group of Eight nations.
Following that tough talk, Harper laid a large bouquet of red carnations along a stone wall near Independence Square, where dozens of Ukrainians lost their lives, gunned down by their own military as they ultimately drove out the country's former regime late last month.
The revolt has sparked a stunning chain of events that saw Russia formally annex the Crimean Peninsula on Friday.
Harper bowed his head at the wall, now serving as a shrine to the victims. He was followed by Foreign Affairs Minister John Baird and Justice Minister Peter MacKay.
Startling scenes of enduring carnage surrounded the Canadians on an otherwise warm and sunny day.
As Ukrainians revelled in the sunshine and strolled through the square, they made their way past makeshift barriers made of tires, bricks and household items, designed to shield demonstrators from rooftop snipers just a few weeks ago.
Heaps of floral bouquets added flashes of colour to the charred pavement and barricades. The flowers marked the spots where Ukrainians died.
Photos of the dead were plentiful.
A mammoth catapult, now shuttered, stood in the centre of the square, aimed directly at a government building.
"It is for Ukrainians, and for Ukrainians only, to decide their future," Harper told a news conference with the interim Ukrainian prime minister shortly before his visit to the square.
"In this principle, Canada will not waver. And to help the Ukrainian people peacefully secure a bright future of freedom, we shall spare no effort."
Harper met with Prime Minister Arseniy Yatsenyuk at the Cabinet of Ministers building on Saturday in the midst of the most serious crisis in eastern Europe since the Cold War.
He gave Yatsenyuk the Ukrainian flag that flew on Parliament Hill during part of the uprising and said free-trade talks between Canada and Ukraine will now be resurrected.
Harper later sat down with the new Ukrainian president at the nearby presidential palace.
Yatsenyuk was effusive in his praise of Canada for its full-throated support of Ukraine during their news conference.
He grew agitated when reminding reporters that Ukraine voluntarily rid itself of its nuclear arsenal years ago, only to have Russian perform an "armed robbery of Ukrainian independence" by making a play for Crimea.
Yatsenyuk also jokingly made a pitch to replace Russia in the G8.
"If the G8 has an empty seat, we are ready to take it," he exclaimed as the assembled media and politicians erupted in laughter.
Harper, meantime, made little secret about where he stood on Russia's membership in the G8.
"I don't think it takes much imagination to figure out what my view is but I will certainly listen to what our partners in the G7 have to say before we arrive at final decisions,” he said.
Canada, which has already given $220 million in aid to Ukraine, also announced it's kicking in $775,000 to help fund a monitoring mission to the country by the Organization for Security and Co-operation in Europe (OSCE).
"For Ukraine, the consequences of the actions of the Putin regime are obvious and can only be remedied by their complete reversal," Harper said.
"All of us who desire peace and stability in the world must recognize that the consequences of these actions will be felt far beyond the borders of Ukraine or even the European continent itself."
His comments almost certainly represented a preview of Harper's message to his fellow G7 leaders at an emergency summit in The Hague on Monday on the sidelines of the Nuclear Security Summit.
"I think it is important that we in the free world not accept the occupation of Crimea, that we continue to resist and sanction the occupation of Crimea and that there be no return to business as usual with the Putin regime until such time as the occupation of Crimea ends," Harper said in Kyiv.
The prime minister's visit to Kyiv comes during an extraordinarily tense time for eastern Europe amid fears that Russian President Vladimir Putin may be pondering flexing his muscles in other countries that were once part of the Soviet bloc.
The two leaders met as pro-Russian forces stormed a Ukrainian air force base in Crimea, firing shots and stun grenades and smashing through concrete walls with armoured personnel carriers. At least one person was wounded, the base commander said.
Russian forces have been seizing Ukrainian military facilities for several days in the Black Sea peninsula, which voted a week ago to secede and join Russia. Harper and his western allies have called that referendum an illegitimate vote.
Harper's Ukrainian counterpart agreed that a strong international response to Russia's actions in Crimea was crucial.
"Russia violating international agreements, Russia making an armed robbery to Ukrainian independent territory, undermined global security," said Yatsenyuk.
"It's up to the UN and a number of international organizations to find the way how to contain those who violate the deals, who breach the deals and who invade the neighbours."
Harper is expected to deliver his account of the situation on the ground in Ukraine to his G7 colleagues, and to push them to take a tougher stand. Germany is of particular concern given that country's close economic ties to Russia.
The head of the Ukrainian Canadian congress, who was in Kyiv with Harper, said he thought the prime minister had significant sway with the G7.
"I think he is highly recognized within the leadership of the G7, we've seen the kind of influence he's had even with President Obama," said Paul Grod.
"We hope to see that when he meets with the G7 on Monday that he'll be able to impress upon him personal first hand discussions and view of the situation in Ukraine, and convince him to in fact push Russia out of the G8 and make it a G7."
Also on HuffPost