The yellow and blue banner fluttered outside schools, hospitals and many shops to mark "Unity Day," a holiday President Volodymyr Zelenskiy created this week after Russia massed troops near Ukraine's borders.
Russia has denied it will invade, but has warned it could take unspecified "military-technical" action if its security demands, including restrictions on NATO, are not met.
On Kyiv's main Khreshchatyk boulevard, where flags decorated government offices, it was business as usual for many.
"Just a normal day, but these flags are here for a purpose, to show we fear no one. They did not scare us," said Mykola, who operates a small coffee stand in the capital.
A loudspeaker at a local government office blared patriotic songs, while television and government Youtube channels broadcast speeches and rousing reminders of Ukraine's nationhood.
"Everyone wants to scare us and we are here to stay," said Ludmila, a pensioner, who wore a tiny Ukrainian flag in the lapel of her coat.
At Kyiv's Olympic stadium, dozens of people unfolded a 200 metre-long Ukrainian flag, waving it to the beat of patriotic music.
In a televised address, Zelenskiy said Ukrainians were united around a common desire "to live in peace, happily, in a family, children with parents."
"No one can love our home as we can. And only we, together, can protect our home," he said. Zelenskiy was later due to inspect military drills in Rivne and fly to the eastern Donbass region to meet with serving troops.
Zelenskiy, himself an active social media user, called on Ukrainians to post pictures and videos of the Ukrainian flag and add hashtags. On Instagram, users posted under #UnityDay.
"Today we have proved, that we Ukrainians are a united nation, united people," said Pavlo Horinov, a official with the state Institute of Family and Youth Policy.
Zelenskiy has long said that, while he believes Russia is threatening to attack his country, the likelihood of an imminent invasion has been overstated by Western allies, responding to Moscow's efforts to intimidate Ukraine and sow panic.
Mykhailo Podolyak, an adviser to Zelenskiy's chief of staff, said this week the president had chosen February 16 as the patriotic holiday partly ironically, in response to media reports an invasion could happed on Wednesday.
The Russian defence ministry said it was returning some troops to their bases after exercises. The United States however, said there were signs more troops were on their way.
US President Joe Biden said on Tuesday that more than 150,000 Russian troops were currently deployed near Ukraine.
Hours after Moscow's announcement of a withdrawl, Ukraine said defence ministry online networks and two banks were overwhelmed by a cyber attack.
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