MOSCOW — Charismatic leaders of the continuing political protests here were detained by riot police Monday night when they refused to leave Pushkin Square after an opposition rally, marking a distinct escalation in tactics by both sides.

For the protesters, it was a test of their commitment to keep up their struggle against Vladimir Putin, despite his winning a huge majority of the nation’s votes in Sunday’s presidential elections. And for Putin it was a measure of his strength and intentions.

During a long winter of street demonstrations calling for honest elections and a law-abiding government, police were tolerant and protesters obediently dispersed when time was up. But a day after Putin’s electoral landslide, hundreds of heavily armored police surrounded the square, and the new young faces of opposition politics stood their ground in a confrontational manner.

After an hour’s standoff in the deepening chill, police bundled Alexei Navalny, Sergei Udaltsov, Ilya Yashin and others into a waiting police van — all of them prominent young leaders of the movement. Navalny, a crusading blogger with a knack for coining such phrases as “the party of crooks and thieves” to describe Putin’s United Russia, is a hero to many, a worrisome demagogue to others.

Police said about 250 other people were detained Monday night in separate political protests around Moscow. Opposition leaders believe the number is significantly higher.

This wasn’t Navalny’s first run-in with the police. He was arrested while leading an impromptu demonstration the day after disputed parliamentary elections in December. While he sat in jail for 15 days, his case helped spark the mass protests that have shaken the capital ever since.

He and the others taken into custody Monday night may have been betting that their actions would have the same galvanizing effect again. He turned out a steady stream of tweets and cell-phone photos from the police van that he was pushed into. But hours later, he was released, even as others were held. Coming off a landslide victory, Putin has taken a tougher approach, but the authorities may have chosen not to make a martyr of Navalny.

The conduct of the election that handed Putin the presidency came in for strong criticism from international observers Monday, while Putin was quoted as toasting his win as an event that will draw Russia together. The U.S. State Department endorsed the criticism, but at the same time said it looked forward to working with “the president-elect” after he is sworn in on May 7.

In an apparent attempt to reach out to Putin’s foes, his protégé, the lame duck President Dmitry Medvedev, on Monday promised a government review of the conviction of former oil magnate Mikhail Khodorkovsky, who has become a political prisoner of sorts, and of the decision that prevented a liberal political party from fielding candidates in Sunday’s election. But no one knows how far those reviews will go.

One indication of the government’s intentions emerged soon after Medvedev made his promise, in Putin’s home city, St. Petersburg, where police arrested 300 demonstrators out of a crowd of about 2,000 who gathered for an unsanctioned protest at St. Isaac’s Square.

Then came the showdown at Pushkin Square in Moscow. The mood within the crowd, variously estimated at between 5,500 and 20,000, was considerably sharper than it had been at earlier, festive rallies. On the spot where historic protests took place in the late Soviet era, demonstrators shouted, “Russia without Putin,” as they have before, but also, “A bunk for Putin’’—meaning a bunk in a prison cell—and “A thief’s in the Kremlin.” At Navalny’s urging, they also chanted, “We’re in charge here.”

After he and the others were arrested, police herded the remaining crowd into a nearby subway station, where they refused to move further and kept up the “Russia without Putin” chant.

The center of Moscow was well fortified Monday evening, with about 12,000 police, interior ministry troops and riot police on duty, police said. Troop trucks and police vans lined many streets.

“If it was a free election, why have they flooded the entire city with troops?” Udaltsov said to the Pushkin Square crowd.

Now that Putin has been declared the winner, the protest movement will have to work out what it hopes to accomplish, and for many that’s difficult to envision.

“I don’t know what we should do next,” said Dmitry Morgachev, 40, a telecommunications engineer who was at Pushkin Square. “Our country has this bad karma – very bad. What happened here in the 20th century can’t be gotten rid of so easily.”

“I’m afraid of blood, of provocation,” said Kirill Dobrov, 32, a sales manager from a Moscow suburb. “I’m scared to think about the future. But we have to support our movement for honest elections. Even if we’re a minority, they must listen to us.”

He and others argued that Putin’s majority was not genuine – that the whole election campaign was so unfair that the reported results are misleading. In this, they were backed Monday by the observer team sent to Russia by the Organization for Security and Cooperation in Europe.

“There was no real competition, and abuse of government resources ensured that the ultimate winner of the election was never in doubt,” said Tonino Picula, one of the leaders of the OSCE mission. “These elections were unfair.”

The campaign beforehand was “skewed” in favor of Putin, said Tiny Kox, head of a delegation from the Council of Europe’s parliamentary assembly. And OSCE observers rated the vote-counting procedures at one-third of the polling places they monitored as “bad or very bad,” said Heidi Tagliani, a co-leader of the OSCE mission.

Researcher Natasha Abbakumova contributed to this story.