The EU and US are to impose sanctions against Belarus over an election that gave President Alexander Lukashenko a landslide victory and third term.
The White House has joined EU leaders, meeting in Brussels, in condemning an overnight crackdown against opposition protesters in the capital, Minsk.
The opposition says 500 people were detained after riot police broke up a five-day long protest against the poll.
Belarus officials insist that last Sunday's vote was democratic.
Opponents have accused the government of vote rigging and international observers said the poll was severely flawed.
The main opposition leader, Alexander Milinkevich, has vowed to continue his fight to have Mr Lukashenko's re-election annulled and said a major rally would go ahead in the capital on Saturday.
White House press spokesman Scott McClellan said the Bush administration would apply additional "targeted sanctions" against some governmental officials in Belarus.
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.
The boyfriend of one of the protesters detained on Friday told the BBC he had received text messages after her arrest.
Alex Tsitovich said he received a message from his girlfriend, Ina, at 0319 saying: "We've been arrested. Everything is going to be alright. I love you."
The next message was sent from the prison in Okrestin, where she said they had been made to stand in the courtyard in freezing temperatures.
"We are already in Okrestin, standing outside in freezing weather, everything is ok," it said.
Mr Tsitovich said a friend had also received messages from his wife who had been taken to Okrestin, where "more than 500 people" were in the yard. Her last text said their mobile phones were being confiscated.
Also among those arrested was the former Polish ambassador to Belarus, Mariusz Maszkiewicz, a Polish embassy spokeswoman said.
Mr Maszkiewicz said he and others in the same police truck were severely beaten.
The BBC's Emma Simpson in Belarus has been on the outskirts of Minsk at a detention centre where the protesters are being held.
From first light, anxious friends and relatives, many in tears, waited at the gates for any news of their loved ones, our correspondent says.
Those under 18 have reportedly been released, but many adults have already appeared in court and been handed sentences of up to 15 days in prison.
'Determined to continue'
Saturday's planned protest marks the anniversary of a short-lived 1918 declaration of the first independent Belarussian state - a traditional day for opposition rallies.
The UK's ambassador to Belarus, Brian Maurice Bennett, told the BBC that EU ambassadors would be joining the protest to observe how demonstrators were treated.
"I think the country stands in the middle of a process which is moving away from the liberal democracy that we call upon Belarus to observe, and that Belarus has itself signed up to," he said.
The OSCE, the organisation which monitored the election and declared it flawed and unreliable, has called for Belarus to respect the rights of the opposition.
Russia, which has not criticised the election, has accused the OSCE of inflaming tensions.