For the past three weeks, the Mariinsky Ballet, one of Russia’s most renowned companies, led the dance world in showing how ballet could return to the stage.

It hosted galas at its St. Petersburg theaters, featuring solos and duets performed by dancers who had undergone weekly tests for coronavirus.

More ambitiously, it had begun staging full-length ballets, with a run of the Romantic classic “La Sylphide.” Audience members were provided with masks and gloves, and seating was distanced, with an empty space between each viewer.

Then, on Aug. 13, the performances stopped. In a development that will concern other dance companies hoping to return to the stage, the Mariinsky Ballet has suspended all performances, classes and rehearsals, a spokeswoman said in an emailed statement. (The Mariinsky’s opera and orchestral programs continue uninterrupted, the statement added.)

She did not answer questions about the reason for that suspension. But on Monday, Interfax, the Russian news service, reported that about 30 people in the company had contracted the coronavirus. Xander Parish, a British dancer who is a principal soloist with the Mariinsky, confirmed in a telephone interview that there had been an outbreak.

“They’ve tried really hard to be safe,” he said. “It’s not like our rehearsals have been badly organized or anything.”

Last week, Mr. Parish said, a member of the company had an elevated temperature, “then another and another and it was like, ‘Oh, dear.’”

On Saturday, everyone in the company was sent an email telling them to quarantine until they could be tested by Russian health authorities, he added.

The news of the outbreak has caused concerns among other European ballet companies, who had been watching the Mariinsky’s return to the stage closely. “We were so full of hope and this is a scary situation,” Christiane Theobald, the acting artistic director for the Staatsballett Berlin, one of Germany’s major companies, said in a telephone interview.

The news, Ms. Theobald said, would not change her company’s plan to hold a gala on Aug. 27, which will feature a maximum of six dancers onstage at a time, all staying at least three meters, or about 10 feet, apart. The Mariinsky’s experience showed that “testing once a week is not enough,” she said, adding that the Staatsballett could not afford to test its dancers every day.

The Mariinsky is not the only Russian company to have been affected by coronavirus. The Bolshoi Ballet, in Moscow, had a positive test in its corps de ballet this month, Makhar Vaziev, its ballet director, said in a telephone interview. When that dancer became ill, the company sent home 54 people who had been in classes with her, Mr. Vaziev said. They all later tested negative for the virus. “Thank God everything is fine,” he added.

The Bolshoi is now testing its dancers weekly, urging them to limit contact with one another and ordering them to wear masks when outside the studio. The company intends to return to its theater on Sept. 10 with four new commissions, followed by “Romeo and Juliet” on Sep. 15. Those programs will involve contact.

Mr. Vaziev said that the company would cancel shows if there was an outbreak. But, he added, he understood that dancers need to work. “The longer dancers don’t have a chance to be onstage, the more they lose,” he said.

Europe’s dance companies, many financed by their governments, are far ahead of American ones in returning to the stage. Several major companies in France, Germany and Austria restarted classes in May as lockdowns were eased, with some now gearing up for shows before socially distanced audiences to mark the beginning of the fall season. Those productions are scheduled to occur even as coronavirus cases are swerving upward across Europe.

In Russia, where President Vladimir V. Putin in July declared the battle against the coronavirus won, worries about the pandemic have receded in recent weeks, with bars and the subway in Moscow crowded. On Aug. 11, it became the first country in the world to approve a coronavirus vaccine despite global health bodies saying it had yet to complete clinical trials.

Mr. Parish said that Mariinsky dancers had returned to class at the end of May. Class sizes were initially limited to just three dancers, with a teacher and a pianist. “It felt brilliant being back, and being able to move again,” he said.

He also felt safe, he said, given the safety measures being taken. Dancers were at first tested every two weeks, then weekly. Rehearsal rooms were disinfected between classes. Dancers also had to undergo temperature checks to get into the building, and wear masks until they entered rehearsal rooms. Outside the Mariinsky, the dancers had no restrictions; now, once they test negative, they are allowed out only for essentials.

When the company decided to hold galas, it split 16 soloists into pairs so they could dance together. Those performances, at the start of August, went well, Mr. Parish said, and were met with applause. That seemed to give the company the desire to “put on the accelerator,” he said. The corps de ballet soon joined rehearsals, and the class size jumped to 30 dancers. “Now this has happened,” Mr. Parish said, “and it took, what, two-and-a-half weeks to kick off?”

The Mariinsky had received “a lot of flak on social media” for returning too soon, he said. But he did not think that was fair. The ballet had taken every measure it could, he said. “And those same people are saying we should wait two years to get back onstage,” he added. “That’s ridiculous. A dancer’s career is 20 years.”

Mr. Parish dismissed criticism of Valery Gergiev, the head of the Mariinsky and a vocal supporter of Mr. Putin. “I respect his desire to get back onstage despite the situation,” he said. “It really shows how much he cares about this art form.”

But Mr. Gergiev has also been questioned by some for taking the Mariinsky’s orchestra on tour, despite the ballet company’s outbreak. Kay Baburina, who runs the publishing department of the Samara Opera and Ballet Theater, said in an email that he and some of his colleagues were “scared of having to contact with the Mariinsky artists” when they played the theater in Samara, Russia, on Aug. 17.

“Going on tour knowing that there is an outbreak in your theater is just too irresponsible,” he said.

In a telephone interview, Olga Smirnova, a principal dancer with the Bolshoi Ballet, said she felt sad about the situation at the Mariinsky, but it had not put her off wanting to return to the stage. “What can we do?” she said. “I think art is more powerful than fear.”

The question of how to come back to stage safely is one that many companies across Europe will soon have to grapple with. Ms. Theobold said she was talking with Berlin health authorities about how to dance safely, but was considering asking her dancers to quarantine in groups for up to a month at a time so they could dance without distancing.

“It’s finding the balance,” Mr. Parish said. “I guess we’re the guinea pigs for that.”