KYIV, Feb 25 (Reuters) - Ukrainian President Volodymyr Zelensky vowed on Friday to stay in Kyiv as his troops battled Russian invaders who are advancing toward the capital in the biggest attack on a European state since World War Two.

Russia launched its invasion by land, air and sea on Thursday following a declaration of war by President Vladimir Putin. An estimated 100,000 people fled as explosions and gunfire rocked major cities. Dozens have been reported killed.

U.S. and Ukrainian officials say Russia aims to capture Kyiv and topple the government. Russia on Thursday seized the Chernobyl former nuclear power plant north of Kyiv, along the shortest route to the capital from Belarus, where Moscow has staged troops.

"(The) enemy has marked me down as the number one target," Zelensky warned in a video message. "My family is the number two target. They want to destroy Ukraine politically by destroying the head of state."

"I will stay in the capital. My family is also in Ukraine."

Putin says Russia is carrying out "a special military operation" to protect people, including Russian citizens, subjected to "genocide" in Ukraine - an accusation the West calls baseless propaganda.

Asked if he was worried about Zelensky's safety, U.S. Secretary of State Antony Blinken told CBS: "To the best of my knowledge, President Zelensky remains in Ukraine at his post, and of course we're concerned for the safety of all of our friends in Ukraine - government officials and others."