Police and demonstrators have clashed during a large opposition rally over disputed election results in the Belarus capital Minsk.
Alexander Milinkevich addressed thousands in a Minsk park
Opposition leader Aleksander Kozulin was among several protesters arrested as they marched on a jail where other demonstrators are held.
There are reports police used tear gas to disperse the marchers.
A peaceful rally was moved to a city centre park after police closed October Square, site of earlier protests.
"The more the authorities conduct repression, the closer they bring themselves to their end," the main opposition leader Alexander Milinkevich earlier told the several thousand who gathered in Yanka Kupala park.
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.
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, but police used megaphones to tell protesters it was closed while diggers cleared away ice from the area.
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.
Despite being confronted by police they stood their ground, shouting slogans and waving flags.
There were unconfirmed reports that tear gas was used to disperse the crowd.
One woman was filmed being beaten by several officers, while another protestor was left almost unconscious and bleeding on the ground, says the BBC's Emma Simpson in Minsk.
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.
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.
There were initial reports that Mr Milinkevich had also been arrested, but his officials later confirmed it was one of his aides who had been detained.
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.