Hundreds of demonstrators rallied outside Moscow's diplomatic mission to demand the release of Nadiya Savchenko, a 34-year-old pilot who is on trial in Russia for the killing of two journalists.
Savchenko announced she was going on hunger strike on Thursday, rejecting both food and water, to protest delays in her controversial trial.
The protesters in Kiev also attached a makeshift gallows to the embassy fence, hanging an effigy of Russian President Vladimir Putin with a sign "murderer," an AFP correspondent reported.
Several embassy windows were broken during the protest.
"The situation is rather tense," Oleg Grishin, spokesman for the Russian embassy, told AFP.
The incident came after around 10 people attacked the mission overnight, smashing several cars, apparently with a hammer, officials said. Grishin said they had thrown flares and smoke pellets.
Ukrainian police said a "hooliganism" probe was opened over the nighttime attack.
Earlier Sunday more than a thousand people turned up on Kiev's iconic Maidan square, shaming Putin and praising the pilot.
Savchenko is demanding that she be repatriated to Ukraine after a judge in the southern Russian town of Donetsk on Thursday unexpectedly postponed her final address to court as her trial nears an end.
Savchenko is seen by her compatriots as a symbol of resistance against the Kremlin accused of fuelling a conflict in eastern Ukraine which has claimed more than 9,000 lives since April, 2014.
Her supporters are concerned that by refusing to drink water she may damage her health irreparably or not live long enough to attend the next hearing set for Wednesday.
Some demonstrators held her portraits, while others held placards reading "Free Savchenko" and "Cannot break Nadiya."
Savchenko's sister Vira, who attended the rally with their elderly mother, said it was hard forUkrainian President Petro Poroshenko to fight "such a mean enemy that is the Kremlin."
"Nadiya went all in. This is not a bluff and not a political project," she said.
Others, like Viktoriya Ivanenko, a 33-year-old university lecturer, expressed fears that the aviatormay die.
"For me Savchenko is the face of Ukraine's liberation struggle and resistance."
In Russia, one of Savchenko's lawyers, Nikolai Polozov, said that she was in "satisfactorycondition."
Writing on Facebook, he said she was being monitored and there was no need to force-feed her.
Refusing both food and water is known in Russia as a "dry hunger strike" and was a method of last resort for some Soviet dissidents under Communism.
Polozov said that Russian human rights ombudsman Ella Pamfilova had informed Savchenko's sister Vira of her condition after her representatives visited her in a detention centre.
Savchenko has fasted before to protest the accusations against her but has never before refused both food and water.
Prosecutors say Savchenko was involved in the killing of two Russian journalists in eastern Ukraine in 2014 in her capacity as a volunteer in a Ukrainian battalion.
She denies the charges and says she was kidnapped and smuggled into Russia.
Putin's spokesman Dmitry Peskov declined this week to comment on Savchenko's hunger strike, saying the issue was not on the Kremlin agenda.
The US State Department has reiterated its call for Savchenko to be released "immediately."
