Biden denounces Russian invasion, casting it as part of a decades-long attempt to crush democracies.

WARSAW — President Biden delivered a forceful denunciation of Vladimir V. Putin’s invasion of Ukraine on Saturday, declaring “for God’s sake, this man cannot remain in power” and casting the military clash in Europe as the “test of all time” in a decades-long battle to defend democracy.

In a speech from a castle that served for centuries as a home for Polish monarchs, Mr. Biden described the face-off with Mr. Putin as a moment he has long warned about: a clash of competing global ideologies, of liberty versus oppression.

Russia’s choice to wage war without justification or provocation was “an example of one of the oldest of human impulses: using brute force and disinformation to satisfy a craving for absolute power and control,” Mr. Biden declared before a crowd of hundreds of people in the courtyard of the Royal Castle and several thousand more outside its stone walls, watching on a large screen.

“We need to be clear: this battle will not be won in days or months either,” the president said. “We need to steel ourselves for the long fight ahead.”

Administration officials have been careful not to hint at Mr. Putin’s removal from office, knowing that it would be taken by the Kremlin as a dangerous escalation. Shortly after Mr. Biden’s speech concluded, the White House insisted that the president was not calling for regime change with his comment about Mr. Putin remaining in power, which appeared to be ad-libbed.

“The president’s point was that Putin cannot be allowed to exercise power over his neighbors or the region,” a White House official said in a statement to reporters. “He was not discussing Putin’s power in Russia, or regime change.”

But some experts said Mr. Biden may come to regret the comment.

“The White House walk back of @POTUS regime change call is unlikely to wash,” Richard Haass, the president of the Council on Foreign Relations, said in a tweet. “Putin will see it as confirmation of what he’s believed all along. Bad lapse in discipline that runs risk of extending the scope and duration of the war.”

The Kremlin spokesman Dmitri S. Peskov told Reuters: “That’s not for Biden to decide. The president of Russia is elected by Russians.”

Mr. Biden unleashed an angry tirade against Mr. Putin for having the “the gall” to say his invasion is intended to “de-Nazify” Ukraine. Mr. Biden called that claim “a lie,” noting that President Volodymyr Zelensky is Jewish and his father’s family was killed in the Holocaust.

“It’s just cynical. He knows that. And it’s also obscene,” Mr. Biden said.

Hours earlier, Mr. Biden reacted emotionally following a brief interaction with refugees from Ukraine who had come to Warsaw. After holding a small Ukrainian girl in his arms for a selfie, the president responded to a question from a reporter by calling the Russian leader “a butcher” for the crushing shelling of Mariupol, the eastern city in Ukraine which has been largely demolished by Russian forces.

Administration officials had billed Mr. Biden’s address as a “major speech” at the conclusion of three days of presidential-level diplomacy in Europe, and the president seemed eager to speak more bluntly than is common in diplomatic venues.

In his telling, Mr. Putin was “bent on violence in the start,” later adding, “It’s Vladimir Putin, who is to blame. Period.” More broadly, he accused Mr. Putin of reviving the Cold War that pitted America against the Soviet Union for decades in a perilous face-off between nuclear-armed powers.

“The forces of autocracy have revived all across the globe,” Mr. Biden said. “Its hallmarks are familiar ones: Contempt for the rule of law, contempt for democratic freedom, contempt for the truth itself.”

Mr. Biden’s statement that Mr. Putin could not remain in power could be perceived “as a call for regime change,” said Michal Baranowski, a senior fellow and director of the Warsaw office of the German Marshall Fund, a nonpartisan policy organization. But he said the comment was unlikely, in itself, to lead to any escalation.

“Russia is responding to military logic much more than political logic,” Mr. Baranowski said, noting the powerful explosions near Lviv heard minutes before Mr. Biden began speaking.

In his address, Mr. Biden drew a familiar line in the sand, saying American troops will not go into Ukraine, which is not a NATO member, while also issuing a stark warning to Mr. Putin.

“Don’t even think about moving on one single inch of NATO territory,” Mr. Biden said, raising his voice in one of the more animated moments of his address.

He described the war in Ukraine as nothing less than an extension of the Soviet Union’s long history of oppression of countries like Hungary, Poland and Czechoslovakia. Those countries eventually won their freedom, he said, but said “the battle for democracy did not conclude with the end of the Cold War.”

“Today, Russia has strangled democracy and sought to do so elsewhere,” he said.

Mr. Biden repeated what has become the rallying cry of his foreign policy, framing Russia’s invasion of Ukraine as part of a broader global “battle between democracy and autocracy, between liberty and repression.”

The United States and its allies have unified behind economic sanctions that have crippled Russia’s economy.

Mr. Biden has been pressed in recent days for stating that sanctions were never meant to deter the invasion — despite his top aides saying the opposite last month — but rather are intended to hurt the Russian economy and hobble Mr. Putin’s ability to wage war in the long term.

Mr. Biden raised his voice multiple times in the speech, but he also quickly shifted to a familiar quiet, empathetic tone as he described the plight of the millions of refugees who have fled Ukraine, some of whom he met earlier in the day.

“I didn’t have to speak the language or understand the language to feel the emotion from their eyes,” he said, and he made reference to his own personal tragedies.

“Let there be no doubt that this war has already been a strategic failure for Russia,” Mr. Biden said. “Having lost children myself I know that’s no solace to the people who have lost family.”