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 200,000 — 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 led 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 coming to power.
The protest had a festive air, in stark contrast to the tense moods 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.
For the first time, Belarusians were allowed to walk freely in the city center, wrapped in opposition flags and chanting antigovernment slogans. After gathering near the obelisk, they walked toward the main square, blocking traffic on the capital’s main avenue. Only one week ago, a group of clapping people on a sidewalk would have been violently dispersed by the riot police. On Sunday, the police were nowhere to be seen.
Many protesters said they turned out because they did not expect to be violently dispersed. Others said they came because they were shocked to learn that protesters had been tortured after being detained at previous rallies.
“These events united everybody,” said Olga V. Golovanova, an economist. “We have woken up to the fact that we want to be free, we want to he human,” she added. “The government believed that they are gods and we are nothing.”
Indeed, in a speech to supporters shortly before city streets filled with people demanding that he step down, Mr. Lukashenko equated his own fate with that of the nation, saying, “If you destroy Lukashenko, it will be the beginning of the end for you.”
Protesters relished the opportunity to speak their minds freely. They took selfies in front of the main security service building, still called the K.G.B. in Belarus, something that was unimaginable just days ago.
“I haven’t seen anything like this before,” said Vladislav A. Ianovich, 18, a computer science student standing wrapped in a European Union flag. “I think we need to repeat such rallies several times and the country will change. It has already changed.”
For some, however, the opposition’s euphoria seemed premature given that Mr. Lukashenko is still in power.
“This is not the end yet,” said Sergei, 57, a teacher at a state-run institution. He said he feared giving his last name because he wanted to protect his students. “It will all depend on what factory workers will do,” he said.
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 an outside military threat. He and Mr. Lukashenko spoke by telephone on Saturday and again on Sunday.
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 tenure, 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 did offer 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’s staying in power. A prominent pro-Kremlin politician, Konstantin Zatulin, described Mr. Lukashenko last week 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. “Like all Belarusians,” he said, “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 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.