The rights defenders said on Tuesday that the prisoners are being held in crowded conditions, and are being denied water and the right to receive care packages.
The activists were taken to detention at Zhodino, about 50km east of the capital, from Minsk's central square after police broke up a camp of demonstrators campaigning against the results of presidential elections on March 19 regarded by many in Belarus and abroad as fraudulent.
Up to 18 inmates were squeezed into cells designed to accommodate five, the rights advocates said.
"People are protesting [against] the mockery and terrible conditions of confinement in the jail," said Valentin Stefanovich, an activist from the Vyasna rights centre, referring to 20 prisoners who have entered the fifth day of their hunger strike.
The rights supporters said nearly 1000 activists have already been sentenced or are behind bars awaiting sentencing for taking part in protests for supporting the opposition after the election, which returned Alexander Lukashenko to the office of the president for a third term.
Many opposition supporters have
been sent to jails outside Minsk
So many have been picked up in Minsk that many are being sent to jails outside the capital, but the detentions are taking place across Belarus, they said.
The election, which officials say Lukashenko won with 83% of the vote, set off days of demonstrations that drew thousands of people to Minsk's central square.
The protests there ended when police staged a pre-dawn raid on Friday, breaking up an opposition tent camp that had given demonstrators a round-the-clock presence.
Alexander Milinkevich, an opposition presidential candidate who officially received 6.1% of the vote, called the election a fraud.
The US and the European Union said the vote was deeply undemocratic and have vowed sanctions against Lukashenko and other officials.
Milinkevich said 1200 people had been detained in connection with the election. Activists have been sentenced to up to 15 days in jail.
"Belarus had not seen such mass repression," Stefanovich said.
Opposition leader Milinkevich has
called the recent vote 'a fraud'
A group of activists in the Okrestina jail in Minsk demanded on Tuesday that the interior minister, who commands the police force, and the KGB chief resign.
The letter was signed by 500 inmates, including the head of the United Civil Party, Anatoly Lebedko.
An 18-year-old student who was detained in the Minsk tent camp last week said detainees were subjected to psychological abuse.
"We were told we would be taken into the forest and shot, and that girls would be raped beforehand," Polina Denisova testified at Minsk's Lenininsky district court.
Pro-Lukashenko youth activists, meanwhile, again staged rallies outside the American and Polish embassies.