MOSCOW — Russian opposition leader Alexei Navalny called on Russians to take to the streets in mass protests after a court ordered Monday that he be held in custody for 30 days after his return to Russia.

He said Monday’s abrupt court hearing, to which a handful of pro-Kremlin media members were admitted, was a sign of Russian President Vladimir Putin’s fear and weakness.

“They are afraid of you,” he said in a video message after the verdict, calling Putin’s regime a gang of monstrous crooks. “They are afraid, and that is why they do things urgently and secretly.”

“So I appeal to you,” he said. “Don’t keep quiet. Resist! Take to the streets! No one can protect us but ourselves, and there are so many of us that if we want to achieve something, we will achieve it.”

International pressure mounted for Navalny’s release as he was abruptly summoned to the hearing, which he described as “the highest degree of lawlessness.” His attorneys said they were given just minutes’ notice of the hearing.

Navalny faces another hearing Feb. 2 over prison authorities’ claims that he violated conditions set down in a 3½ -year suspended jail sentence relating to a 2014 embezzlement conviction, a case Navalny and the European Court of Human Rights have called a political prosecution. At that point, the original jail term could be reinstated.

Navalny’s Anti-Corruption Foundation announced plans for mass protests across the country Saturday.

“I don’t know what’s going on,” Navalny, 44, said in a video comment in court, where pro-Kremlin media members had been ushered in. He said he thought he would meet his attorneys but found himself suddenly facing a court hearing.

The hearing took place at the Khimki police department near Moscow’s Sheremetyevo Alexander S. Pushkin International Airport, where he was arrested Sunday after flying home to Russia from Germany. He had been receiving medical treatment in Germany after an August poison attack.

“I don’t understand. Why was no one warned? No summons? I have seen a lot of mockeries of justice, but this time the grandpa in his bunker is so afraid that the criminal code has been just torn apart and dumped,” Navalny said, referring to Putin.

In an appeal to the judge, he said it was absurd that only select media representatives were admitted to the court, “but real journalists who are standing outside in the frost are not allowed in.”

Navalny attorney Olga Mikhailova told Judge Elena Morozova that no notice of the hearing had been given to Navalny’s legal team: “It is secret even for me. Having entered here, I found out that a court session was underway. Is this open?” The judge then gave Navalny’s attorneys 30 minutes to read the case material and 20 minutes to consult with their client.

A poster on the wall of the hearing room depicted Genrikh Yagoda, the infamous head of the NKVD, Joseph Stalin’s secret police, from 1934 to 1936. Yagoda supervised show trials and executions of officials during Stalin’s purges.

Dozens of Navalny’s supporters gathered outside the court, chanting “Alexei!” as the temperature dipped to minus-4 degrees Fahrenheit.

Yulia Navalnaya, who returned with her husband from Germany on Sunday, posted a supportive Instagram message to Navalny: “When you were poisoned I wrote that we could cope with everything and we could cope with that,” she wrote, adding the same was true now. “There is nothing we cannot cope with. Everything will definitely be fine!”

Russian authorities have dismissed Western alarm over Navalny’s arrest, amid calls for his release by U.S., British and European officials.

Foreign Minister Sergei Lavrov brushed off the international criticism, describing it as an attempt by Western officials to distract from their own problems.

“It looks like Western politicians view this as an opportunity to divert attention from the deepest crisis the liberal development model has ever found itself in,” he said at a Moscow news conference.

Lavrov said there were no grounds to open a criminal case related to Navalny’s poisoning, saying no poison was found in Navalny’s system when he was hospitalized in Omsk, the Siberian city where his plane landed after he fell ill during a flight to Moscow. He added that German authorities had provided no material evidence of poisoning.

Navalny has blamed the poisoning on Putin and Russia’s domestic security agency, the FSB, a successor to the KGB.

Navalny’s arrest has been condemned by U.S. and European officials, who have called for his immediate release.

“We note with grave concern that his detention is the latest in a series of attempts to silence Navalny and other opposition figures and independent voices who are critical of Russian authorities,” U.S. Secretary of State Mike Pompeo said in statement Sunday.

NATO Secretary General Jens Stoltenberg on Monday joined the calls for Navalny’s release.

British Foreign Secretary Dominic Raab tweeted Monday: “It is appalling that Alexey Navalny, the victim of a despicable crime, has been detained by Russian authorities. He must be immediately released.” Raab added that rather than arresting Navalny, Russia should be explaining “how a chemical weapon came to be used on Russian soil.”

German Foreign Minister Heiko Maas also called for Navalny’s immediate release, saying Russia was bound by its own constitution and respect for civil rights. Charles Michel, president of the European Council, called the arrest unacceptable.

Ursula von der Leyen, president of the European Commission, issued a statement Monday condemning the arrest and calling for Navalny’s immediate release.

“Detention of political opponents is against Russia’s international commitments,” she said.

The French Foreign Ministry said it was following the case with “utmost vigilance” and demanded that authorities release Navalny.

Commenting on Navalny’s arrest, Carl Bildt, former prime minister of Sweden and co-chair of the European Council on Foreign Relations, said, “I guess he is accused of having survived the authorities’ attempt to kill him.” He dismissed Russian authorities’ claim that Navalny’s future would be determined by the courts.

“Is there anyone who believes this statement by the authorities in Russia?” he tweeted. “. . . Everyone knows who decides.”

Amnesty International called for Navalny’s immediate unconditional release, calling him a prisoner of conscience and adding that “Russian authorities have waged a relentless campaign against Navalny.”

The office of U.N. High Commissioner for Human Rights Michelle Bachelet said she was “deeply troubled” by Navalny’s arrest, calling for his immediate release and respect for his rights.

Putin and other Kremlin officials routinely underscore their disdain for Navalny by refusing to ever use his name, resorting to a range of constructions such as “the blogger” or “the Berlin patient” or “the patient who is in a coma for reasons yet to be determined.”

A military laboratory in Germany found incontrovertible evidence that he was poisoned with a Novichok-group agent, a finding confirmed by labs in France and Sweden. Russian authorities deny the poisoning and have blamed Germany.

Europe and Britain have imposed sanctions on Russian officials over the attack.

The investigative reporting group Bellingcat linked Navalny’s poisoning to members of a special unit within the FSB by analyzing flight and cellphone records.

Navalny has accused Putin of direct involvement. Last month, he called a member of an elite FSB toxins unit, Konstantin ­Kudryavtsev, while posing as an official compiling a report for Russia’s National Security Council.

In revelations highly embarrassing to Russian authorities, Kudryavtsev said the team had worked well on the project, explaining that the only reason Navalny survived was because of the actions of the pilot diverting the flight to Omsk and the medical team at the Omsk airport. He described an intense coverup operation that included twice cleaning Navalny’s clothing, in particular his underpants.

Navalny has been under intense security surveillance for years and has faced repeated arrests and harassment, including having his bank accounts and those of his family frozen last year. Authorities recently announced new embezzlement charges against him, meaning he could face a lengthy jail term if convicted.