Thousands of protesters marched through central Moscow on Tuesday under flags from across the political spectrum, defying government efforts to silence them — and fierce thunderstorms besides.

The crowd, estimated by police at 18,000 and by organizers at 50,000, rallied even in the absence of half a dozen protest leaders, who spent much of the day being questioned by investigators pursuing charges of disorder at a May 6 rally. Two prominent organizers were served summonses to appear for questioning Tuesday evening as they stood on a stage to address the crowd.

The marchers appeared more buoyant than chastened. They began gathering at Pushkin Square at noon, and many were soon soaked by the torrential rain. Then, as a hot sun emerged to dry them off, they passed through a wall of metal detectors and walked 11 / 2 miles to Sakharov Prospect, denouncing President Vladimir Putin and declaring Russia free along the way.

‘We won’t be afraid’

An onerous law regulating protests — and setting fines as high as $9,000 for violations — had gone into effect Saturday after parliament passed it quickly last week and Putin signed it Friday. Then came searches of opposition leaders’ apartments early Monday and the summonses Tuesday.

“It only made more people come out,” said Mila Basenko, a 25-year-old architect. “It’s typical, this kind of crackdown. We won’t be afraid.”

She and two young men had fashioned a three-letter word that means “shoo” out of giant pieces of polystyrene foam and colored the letters bright orange, holding them high as they walked along.

“That’s what we want to tell Putin,” said Yaroslav Polyansky, a 24-year-old architect. “It’s short and to the point. Shoo.”

Unlike at the demonstration that turned violent May 6, on Tuesday, the riot police — called cosmonauts for their helmets, face guards and extensive body armor — stayed mostly in buses parked just off the march route until the rally ended. Regular police clustered at intersections, wearing special-occasion white shirts and dress uniforms, with assorted troops in camouflage, olive khaki and gray on the sidelines, away from the protesters but ready to thwart any change of route.

‘Unprecedented conditions’

Tuesday was Russia Day, commemorating the republic’s declaration of sovereignty in 1990, 18 months before the Soviet Union fell apart, and there were shouts in favor of upholding the constitution as people walked slowly along the leafy, 19th-century Boulevard Ring, about a mile from the Kremlin. Nationalists clad in black were accompanied by a bagpiper playing Irish music. Communists flew their red flags, environmentalists their green ones. Libertarians handed out literature. One man had printed “tickets” for rides in prisoner vans, and they became a sought-after souvenir.

From the stage on Sakharov, a lawmaker from A Just Russia asserted that citizens have the right to gather as they choose and to tell the authorities what they think.

“If they start to take the street away from us,” said Gennady Gudkov, “if they put us behind bars and order us to pay fines, we will launch acts of civil disobedience all over the country.”

Sergei Udaltsov, the leader of the communist-leaning Left Front and one of the main organizers, had come to the rally despite an order to appear for questioning Tuesday. His apartment had been searched Monday, and he and liberal leader Boris Nemtsov got the summonses on the stage.

“Our rally is taking place in unprecedented conditions — mass searches, detentions, arrests, criminal cases and these hideous amendments to the law on rallies,” he said.

Udaltsov’s wife, Anastasia, did appear for questioning, as did anti-corruption blogger Alexei Navalny, television personality Ksenia Sobchak and Ilya Yashin.

Yashin, a youthful liberal leader, said he was ordered to return for questioning later in the week. On Tuesday evening, Nemtsov’s apartment was being searched, as was Navalny’s office. Udaltsov’s questioning was postponed until Wednesday. Yashin feared that criminal charges were being prepared against him, Navalny and Udaltsov, which would be sure to inflame the opposition.

A dozen people have been charged with inciting mass disorder May 6 and could serve up to eight years in jail if convicted.

‘Putin’s a thief’

The protests began six months ago over allegations that parliamentary elections had been rigged. The rallies have since broadened in theme, targeting Putin and his supporters.

“Putin’s a thief,” the crowd chanted Tuesday. “Russia without Putin.”

In a statement marking Russia Day, Putin mixed conciliatory rhetoric with strong support for the status quo.

“Everything that weakens the country and splits its society is unacceptable for us,” he said, according to the Interfax news agency. “It is important to listen to and respect each other, to seek mutual understanding and find compromises, to rally society around a positive, creative agenda.”

Elena Volkova, a 50-year-old writer who had come to the rally with her mother, was not reassured.

“We have to be here today,” Volkova said, “so they won’t be coming for us tomorrow.”

By late afternoon, the rally was almost over, but heavy rain and bolts of lightning hastened its close. Double lines of riot police in full armor had appeared at the rear of the crowd, funneling everyone into the metro, preventing any spontaneous strolls toward the Kremlin.

No one was arrested.