Lithuania has protested strongly to its neighbour Belarus over the treatment of two Lithuanian journalists, who were covering an opposition rally in Minsk.
Tuesday's demonstration in Minsk was broken up by police
A statement from its foreign ministry said that the assault and detention of the journalists was "a gross violation of the principle of press freedom".
Dozens of protesters at Tuesday's rally have been sentenced in Belarussian courts for public order offences.
The European Commission has also objected to the authorities' actions.
EU External Relations Commissioner Benita Ferrero-Waldner said in a statement: "It is unacceptable that peaceful demonstrators, including journalists, were reportedly arrested and some subjected to violent treatment."
The unsanctioned rally, attended by around 2,000 people, was held to mark 90 years since Belarus's first declaration of independence - a brief period of freedom from Russian rule.
Lithuania's state broadcaster said that Belarussian police used "brute force" against two of its staff who were filming the demonstration, seizing their material and damaging their equipment.
The broadcaster has demanded an official apology, and the foreign ministry has summoned the Belarussian ambassador to Vilnius.
Lithuania is a strong supporter of the opposition in Belarus, and organised its own events to mark the anniversary.
Opposition demonstrators were defiant when taken into court
Belarus's interior ministry said that 70 protesters were facing public order charges after the demonstration, and the courts had sentenced at least 40 of them, handing down fines or 15-day prison sentences.
The country's opposition said the authorities' actions could signal an end to the efforts of President Alexander Lukashenko to improve relations with the West.
The prominent opposition leader Alexander Milinkevich told the news agency Reuters: "The authorities have done a complete about-turn in tactics. There had been a real attempt to avoid clashes before, but this time they clearly wanted to use force."
President Lukashenko has been courting the EU, in particular, since an argument with Russia last year over energy prices - and the EU had praised Minsk for its restraint at opposition rallies.