Multiple explosions were heard and several fires were burning in the Ukrainian capital, Kyiv, early on Thursday as Russia attacked the city with ballistic missiles and drones.

Many residents of the capital had packed into subway stations with sleeping bags and pets, hunkering down for the night amid warnings that a large-scale attack was imminent.

President Volodymyr Zelensky of Ukraine had urged people to heed air-raid alarms and be “especially careful,” saying on Wednesday evening that intelligence had indicated that Russia was preparing another “massive strike.”

The air-raid sirens wailed in Kyiv just a few hours later, signaling the start of a long and loud night for residents.

Attack drones came first. The rat-tat-tat of air defenses firing was followed by one large boom around 11 p.m., then more. A large fire was soon seen burning in the city center, with a smaller blaze just beyond it.

Mayor Vitali Klitschko of Kyiv implored people to stay in shelters as the night wore on and ballistic missiles entered Ukraine’s airspace. Another series of powerful explosions then started to rock the city just before 2 a.m. Thursday local time, setting off car alarms in the streets.

Rescuers were racing to respond to a collapse at a nine-story apartment building where people were trapped, Mr. Klitschko said later. Firefighters also were working to extinguish blazes in at least two districts of the city, including one on the roof of a hotel and another at a residential building.

The full extent of casualties was not yet known, but Mr. Klitschko said that five medical workers had been wounded.

Many in Kyiv had been bracing for a large-scale assault. Ukraine has been heaping pressure on President Vladimir V. Putin of Russia by launching long-range drone attacks on Moscow, disrupting Russian fuel supplies and pounding Crimea — the peninsula Russia illegally annexed in 2014 — with drones and missiles.

Ukraine says the goal is to take the war to Russia and to get Mr. Putin to agree to end the conflict. But Mr. Putin — even as his ability to isolate Russian society from the war’s effect has diminished — has expressed defiance and dug in.

“Putin wants to keep fighting,” Mr. Zelensky said on Wednesday in Ireland before rushing home. “That is why he must face conditions that make it impossible for him to keep this war going.”