Top opposition Telegram channel, Nexta, says authorities have opened more than 250 criminal cases against President Lukashenko's would-be rivals, activists, bloggers, and ordinary Belarusians.
Belarusians have taken to the streets in support of the ex-Soviet country's political prisoners and against the disputed election, with the latest weekend march coming after Brussels and Washington introduced sanctions against strongman Alexander Lukashenko.
Belarusian police said on Sunday they used water cannon to disperse the demonstrators and made several detentions.
"Water cannons have been used in Minsk," Interior Ministry spokesperson Olga Chemodanova told AFP news agency.
"There are detentions," she added, without providing further details.
Ahead of the latest protest that began at 1100 GMT, the top opposition Telegram channel, Nexta Live, which has coordinated protesters and has more than two million subscribers, urged Belarusians to stage a march in support of "political prisoners."
Nexta said that authorities have opened more than 250 criminal cases against Lukashenko's would-be rivals, activists, bloggers and ordinary Belarusians.
"The authorities have taken these people hostage," it said.
"We have not forgotten them and demand freedom for all political prisoners!"
Dozens of 'political prisoners'
The opposition movement calling for an end to Lukashenko's rule has kept up a wave of large-scale demonstrations since his controversial election win on August 9, with around 100,000 or more people taking to the streets every Sunday.
Russia has backed its longstanding ally Lukashenko, offering financial backing and promising military support if events turn against him.
According to the Viasna rights group, there are now 77 "political prisoners" in Belarus including opposition blogger Sergei Tikhanovsky, who was not allowed to run for president, and opposition leader Maria Kolesnikova, who ripped up her passport to prevent authorities from deporting her.
Also on the list is Belarusian-US strategist Vitali Shkliarov, who has worked on US Senator Bernie Sanders's presidential campaign and advised the Russian opposition.
He was arrested in late July as he arrived to see his elderly parents.
On Friday, the United States and the EU hit Belarus with long-awaited sanctions for rigging the vote and orchestrating the crackdown on protesters, targeting key officials – but not Lukashenko himself.
Minsk swiftly announced tit-for-tat "counter-sanctions" against the EU, although it was not clear what form these would take or what or who they would target.
Earlier this week Belarusian authorities withdrew the accreditation of all foreign journalists.
'Gesture of support'
Over the past weeks, the Interior Ministry has used water cannon, stun grenades, and rubber bullets against peaceful protesters and detained thousands of people, many of whom complained of torture and abuse in custody.
Several people have died in the crackdown.
After the blogger Tikhanovsky was jailed, his wife Svetlana Tikhanovskaya ran in his place and claimed victory over Lukashenko.
After taking shelter in EU member Lithuania, the 38-year-old political novice has been engaged in a diplomatic push to drum up support for the embattled Belarusian opposition.
She has already met French President Emmanuel Macron and will travel to Germany for a meeting with Chancellor Angela Merkel on Tuesday.
'Begin talks with Lukashenko'
Student Dmitry Demeshkevich, who planned to take to the streets on Sunday, said he was aware of the sanctions and Tikhanovskaya's upcoming talks with Merkel.
He said the sanctions were a gesture of support but "nothing more" as it was not clear if the Western punitive measures including travel bans would work.
"It's better to begin talks with Lukashenko over his exit," he said.
"Europe can be a mediator but it's us who should get him to quit."
Lukashenko, who has ruled ex-Soviet Belarus for 26 years, has accused Western countries and NATO of supporting protesters and trying to destabilise the country.
He put his military on high alert after the vote and Belarus this month will host war games with several other ex-Soviet countries including Russia in what is being touted as a show of force against NATO.