The EU's top diplomat warned on Thursday that "we are witnessing a further escalation of repression against the Belarusian people".
More than 200 people were detained across Belarus on Thursday during protests against President Alexander Lukashenko as the country marked Freedom Day.
In the capital, Minsk, it was the first sizeable turnout of demonstrators since weeks of protests rocked the country last year. Freezing temperatures and the authorities' violent crackdown had led to numbers dropping significantly.
Thursday's protest marked the anniversary of the 1918 declaration of the Belarus People's Republic, an independent state that lasted only a few months before the Red Army moved in. The opposition traditionally observes the anniversary as the country's unofficial Freedom Day.
To avoid widescale arrests, small groups marched through courtyards and streets.
Riot police set up a heavy presence in the centre of Minsk and deployed water cannon vans. Videos on social media showed police arresting people holding the red-and-white flags that are an emblem of the opposition to Lukashenko.
Interior Ministry spokeswoman Olga Chemodanova said more than 200 people were detained in connection with unauthorised gatherings throughout the country.
Last year, more than 33,000 people were detained for taking part in protests against Lukashenko and the official presidential election result, which credited the incumbent who has been in power for more than 26 years with over 80 per cent of the vote.
Many of those arrested reported being subjected to physical abuse.
According to the Viasna Human Rights Centre, 302 people are still being held as political prisoners including journalists, activists and members of the opposition.
On Thursday, police also arrested several top figures of an organisation of ethnic Poles in Belarus. The arrests came a day after their leader, Andżelika Borys, was sentenced to 15 days in jail for holding unauthorised mass events.
Authorities on Thursday opened a criminal case against Borys for allegedly inciting social hatred, a charge that could carry a sentence of 12 years. Leaders of the ethnic Poles have organised actions in memory of the anti-communist underground in Poland and Belarus after World War II, which offended Belarusian authorities.
The European Union's top diplomat, Josep Borrell, said on Thursday that members of the bloc "call on Belarus to release Ms Andżelika Borys and Mr Andrzej Poczobut immediately and unconditionally, along with all political prisoners currently detained."
"We are witnessing a further escalation of repression against the Belarusian people, including orchestrated campaigns of persecution of human rights defenders, journalists and civil society in Belarus," he added.