Navalny began serving his initial 30-day term Monday after being detained on his way to a Moscow protest he organised, part of demonstrations held in several cities to protest government corruption.
The authorities detained hundreds of others in the city centre, calling the gathering dangerous and illegal.
"The judge keeps Navalny under arrest for 25 days," a spokeswoman for Navalny, Kira Yarmysh, wrote on Twitter."It has been reduced by five days."
He is now scheduled to be released on July 7.
Navalny, a charismatic 41-year-old opposition leader who plans to stand in next year's presidential elections against Vladimir Putin, has called two major protests this year against government corruption.
The protests Monday had been authorised in some places but banned in others.
In Moscow, Navalny claimed that City Hall prevented his team from hiring a stage and sound equipment for a designated location, prompting him to call upon his supporters to meet elsewhere.
Thousands gathered on the city's Tverskaya Street, but the police swiftly detained hundreds and many were sentenced to fines and jail time in line with the country's harsh legislation criminalising street gatherings.
In Saint Petersburg, at least 50 people were given terms of up to 10 days in jail. Nationwide, more than 1,700 people were detained during the protests, though many were later released without charge.
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