In a joint statement, Ashton and Clinton called for the immediate release of over 600 demonstrators, including presidential candidates, who were taken to custody following the recent presidential elections in Belarus.
Five former candidates and 14 other opposition activists detained for protests over Sunday's vote will remain in custody and may face charges of "organising mass disorder," a policy officer said.
They could face up to 15 years in prison if convicted.
Several hundred Belarussian riot police used batons and shields to disperse a crowd of at least 10,000 protesting at the election of President Alexander Lukashenko for his fourth term and demanding a second round of voting.
Many were beaten and more than 600 activists, journalists and ordinary Belarussians were arrested.
"Respect for democracy and human rights remain central to improving Belarus’s relations with the United States and the European Union. Without substantial progress in these areas, relations will not improve," Ashton and Clinton said.
Russia sent the opposite signal. The Kremlin's ambassador to Minsk said Moscow supported legal action against the leaders of the demonstrations.
"They mainly sought the West's refusal to recognise the elections in Belarus," Alexander Surikov said at a press conference. He added, however, that the remaining "mass" of those detained should be forgiven.
The renewed support of Russia, which declared days before the election that it would abandon duties on oil exports to Belarus, came after a tumultuous year between the two neighbors, marked by frequent mutual bickering.
Belarussian finances hugely depend on its relations with Russia – its largest export market – and the oil export duty deal is to save Belarus some $4 billion (€3 billion) next year.
Among the detained candidates are poet Vladimir Neklyayev, former deputy foreign minister Andrei Sannikov, and a leader of Belarus Christian Democratic party, Vitaly Rymashevsky.
Neklyayev was beaten by the police and detained at the start of the protests. Sannikov and Rymashevsky were also badly beaten, their lawyers said, although Lukashenko said after the election that the police acted within the law.
"When I saw my client he was in a horrible state," Pavel Sapelko, Sannikov's lawyer, told Reuters. "His leg was beaten or twisted, he couldn't move on his own, bruises on hands. He did not receive any medical treatment."
Lukashenko vowed on Monday to thwart any attempt at "revolution" and said there would be no more "senseless democracy" in Belarus after police broke up the protests.
Lukashenko, 56, who has ruled Belarus with an iron fist since 1994, was officially declared victor with nearly 80 percent of the vote. The opposition said the vote was rigged and the real level of support was far lower.
(EurActiv with Reuters)