MINSK, Belarus — Minutes after President Aleksandr G. Lukashenko of Belarus vowed to stand firm against protesters he reviled as “rats,” “trash” and “bandits,” antigovernment demonstrators staged their biggest protest yet on Sunday to oppose a fraud-tainted presidential election a week earlier.
Tens of thousands of protesters — some estimates put their number at well over 100,000 — turned out in the center of Minsk, the capital, dwarfing a rally of Mr. Lukashenko’s supporters earlier in the day.
It appeared to be the largest protest in the history of Belarus, a former Soviet republic that Mr. Lukashenko has ruled since 1994.
As the crowd gathered around a Soviet-era obelisk on Victors Avenue, many chanted for Mr. Lukashenko to leave and waved the traditional white and red flag, which became a symbol of the opposition after the president replaced it with a more Soviet-looking national flag soon after he came to power.
The protest had a festive air, in stark contrast to the tense mood of far smaller rallies last week that were violently suppressed by security forces, leaving at least two people dead, many injured and more than 6,000 under arrest. Riot police officers did not intervene on Sunday, however.
The protest came in response to a call for a “March for Freedom” by Svetlana Tikhanovskaya, the main opposition candidate in the presidential election. She joined the race after the arrest of her husband, Sergei Tikhanovsky, a popular blogger who had planned to run as a candidate. Ms. Tikhanovskaya, who says she won the election, was forced to leave Belarus for neighboring Lithuania early last week.
The mass protest on Sunday suggested that Mr. Lukashenko, who claimed a landslide re-election victory with 80 percent of the vote on Aug. 9, had failed in his efforts to intimidate opponents through a frenzy of police violence and increasingly strident warnings that the unrest could open the way for military action by NATO.
Addressing his supporters, many of them state employees, at an outdoor rally in Minsk on Sunday afternoon, Mr. Lukashenko attacked his opponents with defiant and often crude bravado, insulting his critics, rejecting calls for a new election and accusing NATO of massing on his country’s western border.
Denouncing his foes as traitors “controlled by puppeteers, by outsiders,” Mr. Lukashenko, a 65-year-old former state farm director who is often called “Europe’s last dictator,” warned that “even if they calm down now, they will again crawl out of their holes like rats after a while.”
His claims of a military buildup by the American-led military alliance followed a pledge by President Vladimir V. Putin of Russia that Moscow would support Belarus if it faced a military threat from outside.
In a statement issued on Sunday, the Kremlin said that Russia stood ready “to provide the necessary assistance to resolve the problems that have arisen,” and referred to a collective security treaty signed in the early 1990s by Russia, Belarus and seven other former Soviet states. The treaty stipulates that aggression against one member of the alliance amounts to an attack on all of them.
Mr. Lukashenko seems to have calculated that he can best secure Russian help against his domestic opponents by ginning up a fake military crisis on the border. The Belarusian Defense Ministry said on Sunday that it would hold military exercises near its western border from Monday through Thursday.
NATO’s spokesman, Oana Lungescu, said the alliance “is closely monitoring the situation in Belarus,” but added that “there is no NATO buildup in the region.”
Just weeks ago, Mr. Lukashenko was accusing Russia of plotting to overthrow him. But facing the biggest political challenge of his 26-year rule, he made a U-turn, the latest in many over the years by the highly erratic president, and now looks to Moscow as his best hope of survival.
Whether Mr. Putin, who has increasingly tired of Mr. Lukashenko’s flip-flops and periodic flirtation with the West, wants him to survive, however, is an open question. The Russian leader offered his congratulations on an election victory that European countries and the United States dismissed as fraudulent.
But a Kremlin account of a telephone conversation between the two leaders on Saturday did not include any endorsement of Mr. Lukashenko staying in power. A prominent pro-Kremlin politician, Konstantin Zatulin, last week described Mr. Lukashenko as “deranged” and his re-election as “a total falsification.”
In a sign of growing disenchantment among even government employees, the Belarusian ambassador to Slovakia, Igor Leshchenya, posted a video on YouTube on Sunday expressing support for the protesters, saying, “Like all Belarusians, I am shocked by accounts of torture and beating against my fellow citizens.”
Even state-run factories — once solid bastions of support for Mr. Lukashenko — have tilted toward the opposition, with strikes gathering steam late last week at a number of state-owned industrial enterprises, including a tractor factory in Minsk.
The pro-government rally on Sunday only highlighted Mr. Lukashenko’s shrinking base of support. Many attendees had to be bused in from towns and villages outside the capital. But they included people who voiced genuine support for the president, or at least his promise to keep the country safe from outside aggression.
“The West doesn’t need us,” said Olga N. Mokhnach, 43, a music instructor. For all of Belarus’s economic and other problems, she said, “we are not in the same dire situation as Ukraine,” which toppled its own president in 2014 and is now mired in a grinding war with Russian-armed separatists.
Standing together with her husband, Vladimir, 52, Mrs. Mokhnach said that Belarusian society had largely cracked along generational lines. She said the couple’s two children — ages 14 and 16 — had turned against her and her husband politically.
“We shout at each other every evening,” Mrs. Mokhnach said.