The International Press Institute (IPI) in Vienna, its affiliated South and East Europe Media Organisation (SEEMO), and the New York-based Committee to Protect Journalists (CPJ) called on Minsk to stop trying to intimidate independent reporters.
"We are alarmed at the arrests and jail sentences handed down to journalists," SEEMO Secretary-General Oliver Vujovic said in a statement.
On 3 January, Belarus freed one of five opposition presidential candidates it has held since the election. Police said they expected to release most of the remaining detainees soon. About 200 out of 650 were thought to still be jailed.
But Belarus has also decided to shut down the mission of the Organisation for Security and Cooperation in Europe after OSCE monitors criticised as "flawed" the vote that handed President Alexander Lukashenko, 56, a fourth term in office.
Vujovic expressed special concern about Natalya Radina, editor of the website Charter 97, and Irina Khalip, correspondent for Moscow-based newspaper Novaya Gazeta, whom he said remained behind bars at a Minsk detention facility.
"For us, it is important that in Belarus journalists be able to work independently, professionally and freely, like in other countries."
The IPI/SEEMO statement quoted the Belarussian Association of Journalists as saying 24 journalists had been arrested in the crackdown and 21 were physically assaulted.
Some of the arrested journalists had been sentenced to up to two weeks' detention, while others remained under investigation.
The CPJ called on the European Union to link diplomatic relations with Belarus to the release of jailed journalists.
It said a new campaign by Minsk authorities to promote child safety could in fact cloak efforts to seize the three-year-old son of Khalip and former presidential candidate Andrei Sannikov, also imprisoned, from his grandparents. "The ugly attempt to place Khalip's young son in a foster home against his grandparents' will is an abomination. Such outdated Soviet intimidation tactics have no place in 21st Century Belarus," CPJ Europe stated.
(EurActiv with Reuters.)