Nearly 400 people were arrested during Independence Day celebrations in Belarus this week.
Human Rights Watch said demonstrators in the streets in silent protests against the government were beaten by riot police.
Victoria Nuland, a spokeswoman for the U.S. State Department, condemned the behavior of Belorusian authorities in crackdowns that began when weekly demonstrations started in early June.
"We urge the government of Belarus to release those detained, to respect the rights, including freedom of assembly, of the people of Belarus," she said in a statement. "We continue to call for the immediate, unconditional release of all political prisoners in Belarus."
Washington and its European allies hit Minsk with sanctions after President Alexander Lukashenko rounded up hundreds of his political opponents following elections early this year.
Lukashenko in June threatened to close the country's borders if there were an economic catastrophe. Inflation in the country is in the double digits.