President Volodymyr Zelenskyy said he tried to call Russian President Vladimir Putin late on Wednesday but the Kremlin remained silent.
Mr Zelenskyy made the comments in an emotional video address early on Thursday.
Speaking in Russian hours after declaring a nationwide state of emergency, he said: “The people of Ukraine and the government of Ukraine want peace.
“But if we come under attack that threaten our freedom and lives of our people we will fight back”.
Addressing those in Russia, Mr Zelenskyy said: “We are different, but that is not a reason to be enemies”. “We want to determine our history by ourselves. In peace, calm and honesty".
In an apparent reference to Putin’s move to sanction the deployment of the Russian military to “maintain peace” in eastern Ukraine, Zelenskyy warned that “this step could mark the start of a big war on the European continent”. “Any provocation, any spark could trigger a blaze that will destroy everything,” he said.
He challenged the Russian propaganda claims, saying that “you are told that this blaze will bring freedom to the people of Ukraine, but the Ukrainian people are free”.
The United Nations Security Council quickly scheduled an emergency meeting on Wednesday night at the request of Ukraine, which said it feared an immediate threat of a Russian invasion.
Ukraine’s Foreign Minister Dmytro Kuleba asked for the meeting in a letter to Russia’s UN Ambassador Vassily Nebenzia, who holds the council presidency in February.
The meeting is scheduled for 9:30pm EST (2:30am GMT).
A virtual meeting of G7 leaders is also expected on Thursday, which will be attended by Prime Minister Boris Johnson who is under increasing pressure to go further on sanctions against those supporting the Russian president. Washington confirmed the US president will also participate in the call after Mr Biden increased his economic action against Moscow by targeting measures against the company that built the Nord Stream 2 gas pipeline to Germany.
The Ukrainian government has said it is closing airports in eastern Ukraine from midnight through to 7am because of the confrontation with Russia.
Ukrainian aviation authorities have also declared some airspace in the east to be “danger areas” because of attempts by Russian aviation authorities to seize control of the airspace.
Ukraine acted after Russia issued a ban on civilian air traffic in airspace over eastern Ukraine.
Earlier on Wednesday, the Kremlin claimed rebel leaders in eastern Ukraine asked Russia for military assistance to help fend off Ukrainian “aggression,” an announcement that immediately fueled fears that Moscow was offering up a pretext for war, with the US calling it a deception on Russia's part.
Anxiety about an imminent Russian offensive against its neighbor soared after Mr Putin recognized the separatist regions' independence, and the West responded with sanctions.
The separatists’ appeal comes after Mr Putin sanctioned the deployment of troops to the rebel territories to help “maintain peace” and the parliament granted him permission to use military force outside the country.
In Ukraine, lawmakers approved Mr Zelenskyy’s decree that imposes the state of emergency for 30 days starting on Thursday.
The measure allows authorities to impose curfews and restrictions on movement, block rallies and ban political parties and organisations “in the interests of national security and public order”. The action reflected increasing concern among Ukrainian authorities after weeks of trying to project calm.
The Foreign Ministry advised against travel to Russia and recommended that any Ukrainians who are there leave immediately.
Kyiv reported heavy shelling on the frontline in eastern Ukraine, where Russia-backed rebels have held territory since 2014.
The Ukrainian military said on Wednesday one of its soldiers was killed and six more were wounded. Separatist officials reported several explosions on their territory and three civilian deaths. The Pentagon said the Russian force of more than 150,000 troops arrayed along Ukraine’s borders is in an advanced state of readiness. “They are ready to go right now, should that be the way Mr. Putin wants to go,” press secretary John Kirby said.
The latest images released by the Maxar satellite image company show Russian troops and military equipment deployed within 10 miles of the Ukrainian border and less than 50 miles from Ukraine’s second-largest city, Kharkiv.