The European Union has called for the immediate release of Belarus opposition leader Aleksander Kozulin.
Mr Kozulin was leading protesters on a march when he was arrested
Mr Kozulin was among protesters arrested on Saturday as they marched towards a jail where other demonstrators were being held.
The EU said it was "appalled" and demanded Mr Kozulin and other detainees be freed immediately.
The arrests came after police and protesters clashed during an opposition rally over disputed election results.
Belarus' interior ministry said 52 people were arrested on Saturday, but human rights groups say the true figure was much higher.
One local civil rights group said as many as 600 people have been arrested in Belarus during the last week.
Mr Kozulin is being held at a detention centre in the town of Zhodino, just outside Belarus' capital, Minsk, his wife, Irina, told reporters.
He has not been charged with any offence and is in good health, she said.
Fellow opposition leader Alexander Milinkevich called for Mr Kozulin's immediate release, but expressed reservations about his decision to march on the jail.
He called the decision to march on the jail a mistake, especially after a series of successful protests earlier on Saturday, says the BBC's Emma Simpson in Minsk.
There would be no more protests for one month, Mr Milinkevich said, when Belarus would mark 20 years since the Chernobyl nuclear disaster that blanketed much of the country in radiation.
Belarusian Interior Minister Vladimir Naumov said Mr Kozulin had been arrested because he had called for people to rise up against the state.
Witnesses said Mr Kozulin - who was a runner-up in the presidential election - and several members of his family were pulled from the crowd by police.
Earlier, Mr Milinkevich told several thousand protesters gathered in Yanka Kupala park that the government's handling of the protests would spell its downfall.
"The more the authorities conduct repression, the closer they bring themselves to their end," he said.
Saturday's demonstration was timed to coincide with the anniversary of the declaration of independence of the short-lived Belarussian republic in 1918.
The march was originally due to be held in October Square, the site of earlier protests, but police used megaphones to tell protesters it was closed.
Internal opposition and international monitors have strongly criticised last Sunday's poll, in which President Alexander Lukashenko took 82.6% of the vote, securing his third seven-year term in office.
Belarus insists Mr Lukashenko's win was fair.
Russia has not criticised the election and accused the OSCE, which monitored the vote, of inflaming tensions.
The US and EU have agreed to impose sanctions on Belarus, likely to include a travel ban against Mr Lukashenko.