Russian opposition activist and anti-corruption crusader Alexei Navalny, 38, stands at a court in Moscow on Dec. 30. (Pavel Golovkin/AP)

Russia’s most prominent opposition activist announced on his Web site Monday that he was breaking his house arrest, his latest open challenge to authorities since a Moscow court convicted him and his brother of fraud last week.

Calling the terms of his detention “illegal,” Alexei Navalny, a flamboyant critic of President Vladi­mir Putin, posted a picture of an electronic monitoring bracelet he said he cut off with kitchen scissors. He said he was within his rights to “refuse to fulfill the requirements” of his court-imposed home imprisonment.

Navalny received a 3 1 /2 -year suspended sentence for embezzlement last Tuesday and was ordered to remain under house arrest until his sentence takes effect. In a trial widely viewed as politically motivated, his brother Oleg was sentenced to a 3 1/2 -year prison term.

The verdict, which originally was scheduled to be announced Jan. 15, was abruptly changed with less than 24 hours’ notice and drew about 2,000 protesters to a rally outside the Kremlin last Tuesday night clamoring for a “Russia without Putin” and “freedom” for the brothers. Navalny, who had encouraged his supporters to take to the streets, broke his house arrest to join the crowd but was promptly detained and returned home.

Critics interpreted the authorities’ reluctance to jail Navalny — along with the court’s snap decision to push up the verdict to the day before New Year’s Eve, Russia’s biggest holiday — as a sign that officials feared turning him into a political martyr.

Navalny’s popular influence is considerable in a Russia where the state still commands an overwhelming measure of authority and public support. He led mass demonstrations against Putin in 2011 and 2012 and challenged the Kremlin’s handpicked candidate for Moscow mayor in 2013. Before the date of the verdict was changed, more than 30,000 people had pledged to attend a protest on Jan. 15.

In his notice that he had broken his house arrest, Navalny announced no plans to stage additional protests, citing only plans “to travel from home to the office and back, and spend free time with family.”

Navalny maintains that the court has no right to keep him at home, because he already has been convicted and the verdict has been announced. He added that he still had not received the text of his sentence, hindering his ability to appeal. The deadline for receiving a copy of the verdict, he added, has expired.

Navalny’s attorney, Olga Mikhailova, told the Russian news service Interfax that it is “illegal to keep him under house arrest in the absence of a motivated court judgment,” thus justifying Navalny’s decision to free himself.

Moscow police told Interfax that they would return Navalny to his home if they find he has left his apartment — and if federal authorities instruct them to do so.