"There is a chemical attack alert in Kyiv. Do not go outside!" another one read.
The public air alert online map also displayed Kyiv and other areas as under a nuclear alert. The app also issued false warnings about Russia’s hypersonic Oreshnik missile and alleged street fighting in Kyiv.
Additionally, loudspeakers in the streets blasted a recorded message instructing people to take precautions.
But Ukraine’s Air Force said its air alert app was hacked and that there was no radiation hazard to be concerned about.
"The air alert app appears to have been subjected to enemy influence. There is no radiation threat! Stay calm and follow our updates on our channel!" the Ukrainian Air Force said in a statement posted to Telegram.
Ukraine’s Center for Strategic Communications blamed Russia for the incident, although no group had publicly claimed responsibility.
Separately, Russia’s Emergencies Ministry said early Monday that its mobile application had been hit by a "massive cyberattack" that allowed attackers to send false information to the public.
"Attention! The mobile application of the Ministry of Emergencies of Russia has been subjected to a massive cyberattack, enabling attackers to send false information to the public," the ministry said on Telegram.
Fox News' Simon Owen contributed to this report.