Starting around 22:47 local time, users received false alerts of nuclear, radiation and chemical threats in Kyiv and other areas, plus a warning of street fighting.
One message, written in Russian, referred to Oreshnik, a Russian intermediate-range ballistic missile that can carry nuclear or conventional warheads.
All-clear messages followed around 23:53.
Hours later, Russia's Ministry of Emergencies said its own mobile alert app had suffered a "massive cyberattack" that sent fake information to the public.
Neither side's claims have been independently confirmed, and it is unclear whether the two incidents are linked.
The disruption also reached beyond phones: authorities in the Lviv region reported that some sirens and loudspeakers broadcast a radiation hazard signal, though there was no real threat.
Ukraine's Ministry of Digital Transformation, which supports the app, temporarily disabled it, which carries its own risk, since users could miss genuine warnings during an outage.
Herbert Lin, a Stanford cyber policy researcher, said it is likely the first time an official Ukrainian information app has been hit this way and suspects it won't be the last.
This also appears to be the first hack of either country's official alert app.
Ukrainian officials said the effect was limited. Parliament foreign affairs committee chair Oleksander Merezhko told the BBC the incident caused no panic, partly because the Russian-language message aroused immediate suspicion. Ukraine's Center for Strategic Communications blamed Russia, pointing to that language. Regional leaders urged calm, and the Vinnytsia head warned people not to click a link in one message, which could be phishing. The app's creators later called it a cyberattack and said the service was being gradually restored. Russia did not name a culprit, saying only that "malicious actors" were responsible, and said the app is merely an ancillary news service, with the wider alert system of sirens, loudspeakers, texts, TV and radio unaffected. The Ukrainian alerts came minutes before a local ceasefire around Oleshky in Kherson region ended, while US-mediated talks between the two countries are set to resume this month.
