The EU has called for the immediate release of Belarus opposition leader Aleksander Kozulin.
Mr Kozulin was arrested along with several protesters on Saturday as they marched towards a jail where other demonstrators were being held.
The EU said it was "appalled" and demanded the Mr Kozulin and other detainees be freed immediately.
The arrests came after police and protesters clashed during an opposition rally over disputed election results.
Belarussian Interior Minister Vladimir Naumov said Mr Kozulin had been arrested because he had begun "to call on people to attack state and restricted installations and even for the physical liquidation of the head of state," AFP news agency reported.
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.
His whereabouts are still unknown.
Earlier, the main opposition leader Alexander 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.
We are already in Okrestin [prison], standing outside in freezing weather, everything is ok
Prison trucks, ambulances and vehicles carrying police reinforcements were spotted down nearby side streets before the rally began.
As the protest grew, with many chanting "Shame" and "Long Live Belarus", security forces began hemming them into groups and pushing them away from the square.
At one point some protesters stopped the traffic but were quickly swooped on by the police.
Hundreds of marchers later moved on to a jail where several hundred of their fellow protesters are being held after their arrest in October Square on Friday.
The authorities said in a statement that stones and unidentified explosive devices had been thrown at the police and that eight of them had been injured.
Both the opposition and international monitors have strongly criticised last Sunday's poll that saw President Alexander Lukashenko take 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.
White House press spokesman Scott McClellan said on Friday that the Bush administration would apply additional "targeted sanctions" against some governmental officials.
No time frame was given, but the sanctions are expected to take the form of travel restrictions and some financial penalties.
Earlier, EU leaders issued a declaration saying that the bloc would "take restrictive measures against those responsible for the violation of international electoral standards".
The measures have yet to be agreed, but EU officials said they would probably include a travel ban against Mr Lukashenko.